I did something like this a couple years ago. I downloaded some open data from the Irish government, heritage sites or something like that, put them into a web map for use in Field Maps. I used that map when I was traveling around to keep an eye out for these sites (they were literally everywhere and often unmarked), had a flag I could set to "Visited" from Field Maps and had a URL in the pop-up to launch an S123 form for taking pics. I could've just used FM for the pics but I wanted to watermark them so I went with S123.
@AdrianWelsh mentioned QuickCapture, I've used it as well and it's generally better (particularly great for collecting linear features to track your walking route) but I did like having Field Maps in the mix too.
