Stuff like that is hard and there's no shortage of attempts of trying to categorize everything.
Lots of conlangs try to have as few root words as possible and then build everything by compounding them. Toki Pona is one example.
Yours is different since the second "root" is contextual (and you want to use a single letter so you have less options) but that might give you a direction to start with.
I'm thinking at most a single sound, so possibly a few letters. I'm going to play with it.
Also maybe look into logban. Verbs in that language have very specific and defined noun relations so maybe you could do something like that. IE "A" means it's living and then that defines that the second letter is leg count, third letter is kingdom, etc.
Will do. Thanks!
