Accepted Solutions
Here is what I ended up doing. It wasn't perfect, and it took a lot of figuring out, but the ultimate solution ended up being pretty simple. The source data were all on my local system so I wasn't making so many web requests.
Hillshade
The USGS's 3DEP (3D Elevation Program) 1 arc second DEM was about the right resolution for me. They have a map viewer where you can generate a list of URLs to download TIF tiles for your map extent. I had over 1600 tiles, so I wrote a Python script to download all these.
The Pro GP tool Mosaic To New Raster will create a seamless DEM. Then, I projected that and drew it with multi-directional hillshade symbology. I just used the layer transparency on my layout's white background to lighten it up. I wanted to include terrain, but dial it back since it wasn't central to the purpose of my map.
NOTE: I tried a few times to pre-generate a raster with hillshade symbology so Pro wasn't doing so much processing to symbolize it on the fly. Everything I tried ended up looking blurry, so I just used the DEM rendered on the fly. It took several hours to export my layout, but it worked, so I guess pre-generating a hillshade raster wasn't necessary. This could probably be it's own forum post, but was a dead end for me.
Vector Tiles
I used the Vector Style Editor to create my own style based on the World Navigation basemap data and its default style. Mostly, I needed a few text labels and highway shields to be a larger font size. Here is a good tutorial on how to use it. I The editor saved my modified style to an AGOL item. AGOL says its type is "Tile layer". The size is only 1MB, so I'm assuming it's just the style definition files and maybe some icons (a.k.a. sprites), but not the actual data.
You can add that AGOL item as a map layer in Pro and you have everything from the basemap on your map symbolized the way you want. Note if I made subsequent changes to the AGOL style, I had to restart Pro to get it to pick them up.
As far as I can tell, the style I used has all style classes set to 100% opacity. I'm not sure if there's a better way to do this, but I used the "Multiply" layer blend mode to draw it over my hillshade.
At 100% scale, the multiply layer blend looks fantastic. If you haven't played with it, it is sooooo much better than just setting transparency. That said, not every feature should blend. Zooming way in to a highway shield up in the mountains with a lot of terrain, you can see the hillshade subtly bleed through (it should just be white). Most roads and man-made features aren't running through this kind of terrain, and it's really subtle at 100% view, but it kind of annoyed me.
If I had time, I would create two basemap styles. The first style would include any feature that would use the layer blend transparency (mostly polygon layers like parks and land use). In the style editor, you can toggle the visibility of style classes, so I'd toggle off all other style classes. The second style would be anything that should be totally opaque, e.g.: roads, highway shields, text labels (turning off visibility for all other style classes from the first style). Then, these would be added as separate layers to my map, using the multiply layer blend on only the first style. This is more or less what the Imagery Hybrid basemap does with its reference layer:
One word of caution on this vector tile style approach is that you cannot edit the data. You can do a lot with the styles, but you're locked into the data Esri provides unless you want to create your own (which I didn't). I don't know if styles can include SQL-type queries to modify (or hide) specific features. In for a penny, in for a pound.
At this point, you're done for most normal map production. Because my map was so massive, I wanted to have all data locally on my hard drive. My first attempt at exporting this layout ran for days before I killed it, and I suspect the culprit was the zillions of requests to AGOL for the basemap.
I used the Pro "Download Map" button to download my basemap layer to a local vector tile package (VTPK). That runs for quite a while and will save a vector tile package file under your Pro project's folder and create a new map layer for it. Just like the AGOL basemap, I used the Multiply layer blend to add in the hillshade.
The download map control has a checkbox to include the basemap and a scale drop-down list to choose the most detailed scale you want. For my purposes, the basemap had way too much detail at the scale I was using. I chose to use two scale intervals smaller than that, which cleaned up the map and greatly reduced the amount of data I was downloading. In the vector style editor, I had planned this out ahead of time. For print maps, figuring out the scale you need in advance simplified matters, since there's a lot of scale-dependent stuff in those styles, and I just needed to worry about one scale. Note that the scale level is in the editor's interface:
The other style editor tip I didn't discover soon enough was clicking on a feature on the map takes you right to the style class for it. This saves a lot of wading through Esri's lists of style classes. They're well organized, but still there's a lot to sift through, or at least there was for World Navigation.
NOTE: The download map tool will allow you to download a lot of data, but there is some restriction. As your map extent gets bigger, the larger scale intervals will not be available in the interface to select. My guess is this is designed to protect you from asking for too much data. In the case of my room-sized map, I was able to download it all in one go, so it's not that restrictive. On my first attempt, I was using a larger map scale, and it was not available in the drop-down list for scales in the download map tool. See my comment on this post if you need to download a larger map extent than what the GUI will let you. You have to do it in batches (tiles... of tiles) and the extents cannot have any gaps or overlaps. I put in a code sample to move the map extent to create a non-overlapping and seamless extents for downloading VTPKs.
I know this seems like a lot, but it wasn't that complicated in the end for the scale of what I'm trying to do.
Here is what I ended up doing. It wasn't perfect, and it took a lot of figuring out, but the ultimate solution ended up being pretty simple. The source data were all on my local system so I wasn't making so many web requests.
Hillshade
The USGS's 3DEP (3D Elevation Program) 1 arc second DEM was about the right resolution for me. They have a map viewer where you can generate a list of URLs to download TIF tiles for your map extent. I had over 1600 tiles, so I wrote a Python script to download all these.
The Pro GP tool Mosaic To New Raster will create a seamless DEM. Then, I projected that and drew it with multi-directional hillshade symbology. I just used the layer transparency on my layout's white background to lighten it up. I wanted to include terrain, but dial it back since it wasn't central to the purpose of my map.
NOTE: I tried a few times to pre-generate a raster with hillshade symbology so Pro wasn't doing so much processing to symbolize it on the fly. Everything I tried ended up looking blurry, so I just used the DEM rendered on the fly. It took several hours to export my layout, but it worked, so I guess pre-generating a hillshade raster wasn't necessary. This could probably be it's own forum post, but was a dead end for me.
Vector Tiles
I used the Vector Style Editor to create my own style based on the World Navigation basemap data and its default style. Mostly, I needed a few text labels and highway shields to be a larger font size. Here is a good tutorial on how to use it. I The editor saved my modified style to an AGOL item. AGOL says its type is "Tile layer". The size is only 1MB, so I'm assuming it's just the style definition files and maybe some icons (a.k.a. sprites), but not the actual data.
You can add that AGOL item as a map layer in Pro and you have everything from the basemap on your map symbolized the way you want. Note if I made subsequent changes to the AGOL style, I had to restart Pro to get it to pick them up.
As far as I can tell, the style I used has all style classes set to 100% opacity. I'm not sure if there's a better way to do this, but I used the "Multiply" layer blend mode to draw it over my hillshade.
At 100% scale, the multiply layer blend looks fantastic. If you haven't played with it, it is sooooo much better than just setting transparency. That said, not every feature should blend. Zooming way in to a highway shield up in the mountains with a lot of terrain, you can see the hillshade subtly bleed through (it should just be white). Most roads and man-made features aren't running through this kind of terrain, and it's really subtle at 100% view, but it kind of annoyed me.
If I had time, I would create two basemap styles. The first style would include any feature that would use the layer blend transparency (mostly polygon layers like parks and land use). In the style editor, you can toggle the visibility of style classes, so I'd toggle off all other style classes. The second style would be anything that should be totally opaque, e.g.: roads, highway shields, text labels (turning off visibility for all other style classes from the first style). Then, these would be added as separate layers to my map, using the multiply layer blend on only the first style. This is more or less what the Imagery Hybrid basemap does with its reference layer:
One word of caution on this vector tile style approach is that you cannot edit the data. You can do a lot with the styles, but you're locked into the data Esri provides unless you want to create your own (which I didn't). I don't know if styles can include SQL-type queries to modify (or hide) specific features. In for a penny, in for a pound.
At this point, you're done for most normal map production. Because my map was so massive, I wanted to have all data locally on my hard drive. My first attempt at exporting this layout ran for days before I killed it, and I suspect the culprit was the zillions of requests to AGOL for the basemap.
I used the Pro "Download Map" button to download my basemap layer to a local vector tile package (VTPK). That runs for quite a while and will save a vector tile package file under your Pro project's folder and create a new map layer for it. Just like the AGOL basemap, I used the Multiply layer blend to add in the hillshade.
The download map control has a checkbox to include the basemap and a scale drop-down list to choose the most detailed scale you want. For my purposes, the basemap had way too much detail at the scale I was using. I chose to use two scale intervals smaller than that, which cleaned up the map and greatly reduced the amount of data I was downloading. In the vector style editor, I had planned this out ahead of time. For print maps, figuring out the scale you need in advance simplified matters, since there's a lot of scale-dependent stuff in those styles, and I just needed to worry about one scale. Note that the scale level is in the editor's interface:
The other style editor tip I didn't discover soon enough was clicking on a feature on the map takes you right to the style class for it. This saves a lot of wading through Esri's lists of style classes. They're well organized, but still there's a lot to sift through, or at least there was for World Navigation.
NOTE: The download map tool will allow you to download a lot of data, but there is some restriction. As your map extent gets bigger, the larger scale intervals will not be available in the interface to select. My guess is this is designed to protect you from asking for too much data. In the case of my room-sized map, I was able to download it all in one go, so it's not that restrictive. On my first attempt, I was using a larger map scale, and it was not available in the drop-down list for scales in the download map tool. See my comment on this post if you need to download a larger map extent than what the GUI will let you. You have to do it in batches (tiles... of tiles) and the extents cannot have any gaps or overlaps. I put in a code sample to move the map extent to create a non-overlapping and seamless extents for downloading VTPKs.
I know this seems like a lot, but it wasn't that complicated in the end for the scale of what I'm trying to do.
