Hacking on an :emacs: clojure-ts-mode indentation issue yesterday, I came across https://www.masteringemacs.org/article/lets-write-a-treesitter-major-mode, which has some nice tricks for working on Tree-Sitter modes. It seems like a great getting started guide overall. (I really should have looked for this earlier đŸ™ˆ)
@kommen that's what I used for uiua-ts-mode!