for content, i mostly pirate things, to be entirely honest. torrenting can be good for audiobooks, but for ebooks, searching archive sites like zlib or even just archive.org i find is often fruitful. alternatively, other P2P networks can be pretty good too (i find alot of stuff on soulseek (yes, the music-sharing network), suprisingly).
as for software setup, KOReader like i said in the comment. it’s substantially more “complete” than the kobo base software, in that it’s got a million options & seems to be able to do anything. not that it’s terribly hard to learn, but there’s a “user guide” on the website that’s very straightforward indeed if you’re finding it somewhat daunting. installation is fairly easy (guide for kobo here). the method i used was the “semi-automated” one that was basically just running a script. i should not that the above installs the 3rd party software “alongside” the base software, so one must start the device “normally” first, then launch koreader from a menu button (added by the install script)
not all moulds though, it should be noted