So - one thing that Tropy can do (according to the documentary) is using a controlled vocabulary. Which is nice - because one of the difficulties in having and managing a database is always consistency.
Now I only have to figure out how to a) get a controlled vocab that I want to use to link up with Tropy, and b) figure out what I want from it. One of them is the Getty Art and Architecture Thesaurus, which might be interesting for me. Another is the ICONCLASS system, which I knew was used by the wonderful IMAREAL database. I've taken a look at it and found that it's too large and too complicated to use for my stuff (especially as it contains so many things that I don't have in any way that it would take too long to re-keyword everything) - but during this, I have discovered that there's a bunch of online databases that use it.
So in case you like databases that use an elaborate and detailed system for their descriptions of art (and who doesn't?), here's the ICONCLASS list of databases. Enjoy!