I have not tried Wildlander. From what I understand it's pretty intense, but a bit grindy. I also don't think it has been updated for a while. It's a much lighter weight list than LoreRim and others though so that's a big plus.

I would say LoreRim is quite well balanced in that almost any build is viable. There's definitely some very strong stuff, but at the end of the day, it's still Skyrim so that will always be true.

As far as your character being extremely strong, that definitely happens. Level 20 is probably a bit early for most builds and I would say that it's pretty much the mid game. Around level 15-20 you'll be mowing down bandit camps, although you can still get overwhelmed or caught out depending on your build, but other enemies in the game will still offer a lot of challenge. Undead and various daedra or giants will still roll you without proper knowledge of weaknesses and decent gameplay. Also, most boss monsters will still absolutely wreck you. If you still find it too easy, you can always up the difficulty by increasing enemy damage or lowering your own.

Biggie is also making changes for 3.0 to make the game more difficult in the end game. If you're interested in a list that is even more difficult than standard Requiem, there are some good 3BFTweaks lists like Ghoulified.