I should also probably look at the main themes (Frio, Vier, Quattro, etc.) and make a list of common features all themes need to have to be functional and which elements are really optional, the things that mean a theme either offers more features at the cost of performance or is lightweight or minimalist.
One of the things that makes themes challenging for Friendica is that this platform has a lot of "moving parts" so there's just many, many things to style, a lot of inconsistencies because of different hands over many years, and it gets more complicated if you also want to modify the UI or add new elements.
I think, for now, I'll just submit a PR to make the compose page and advanced composer a feature check instead of just looking for "Frio" and consider the rest after I finish the theme done that I've been trying to complete. If nothing else it's showing me where the points of pain are in the process.