I had no idea the extent that Kobos allow you to do these sort of things. Heavily considering a Kobo as my next e-reader once my PW3 gives ups. Is there a good starting resource for Kobo modifications like you mentioned?
We also have the amazing Plato reader, like koreader but with a snapper UI and made exclusively for kobo and written in Rust by the guy responsible for bspwm.
No ads per se, but a recommendation tile and a kobo plus subscription tile on the main screen that just eats display space for nothing. I mainly use kobopatch to change the epub reader margins, get the page remaining in a chapter and block firmware updates and a few other details. Totally optional, it's just pure nitpicking here. Here is all my kobo modding work in this repo, you can also find what firmware mods are available...