This is all good advice but I would say if you’re willing (and interested) to try a “40s” like board, the corne (at least in 6 column version) is fairly accessible and is very popular so looking for ideas about how to make your keymap won’t be difficult. And also boardsource now has a fully assembled version of the corne (no soldering).
That is to say, cerement described a very reasonable progression, but if you don’t mind feeling like you don’t know how to type for a bit you can jump into anything that is interesting to you, and many folks agree that smaller boards are more comfortable to type on.