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...
1. https://github.com/pgaskin/kobopatch-patches/releases optional but nice to tweak some small ui details and get rid of ads
2. https://github.com/videah/kobo-tailscale to have a secure connection with your calibre-web instance, but you could also use self-signed certs
3. https://github.com/janeczku/calibre-web/wiki/Kobo-Integratio... adding books to calibre-web will cause them to appear on the device the next time it syncs with calibre-web