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.
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 extensible with plug-ins and the UI is entirely user-moddable with Lua scripts. I use several found here: https://github.com/sebdelsol/KOReader.patches.
(Edit to add a few other collections of mods I've seen recommended: https://github.com/SeriousHornet/KOReader.patches, https://github.com/joshuacant/KOReader.patches/tree/main, https://github.com/joshuacant/KOReader.patches/tree/main.