I see that there's a set of ESRI official vector basemaps that is specifically "for export" to take offline. Is it recommended to use those when authoring a webmap that you plan to use offline in Collector? I don't see them when I search the Living Atlas inside the web map "add item" but reading the summary of the item, it sounds like this would be preferred. Since you have to have a basemap in your map, and since you can't yet indicate that you already have a basemap on the device you want to reference when creating an offline area (ad hoc or via web map), there is going to be a lot of basemap downloading going on.
I found them with the tag World_Basemap_Export_v2