I tried to set up KOReader multiple times and always got stuck figuring out where all the settings are and how exactly they work. It seems very complete, but I'm just too dumb to actually use it.

I went through the same on my 10-inch Android tablet. The key is just sticking with it long enough to get it set up for your needs and prefs. I'm glad I did because it's really great. While the docs are quite good, if you get stuck just use an LLM. Once you get it configged, you won't be using the UI much.

KOR is both complete and very capable. It can very probably do everything you'll ever want but, as many people observe, the UI is just a bit weird. That's not to say the UI is broken or inconsistent, its simply that its approach is a little different than you'd expect. Once I got familiar with the main flow of the UI, it hasn't been an issue for my daily-use needs. If you care about flexibility, control, features and having it "your way", I think it's well worth the one-time effort to set up. It's also extremely user-moddable with Lua scripts. I use several found here: https://github.com/sebdelsol/KOReader.patches.