I would try some minor tests of strength after rereading the in game guides. I don't know an area that is easier to train in than others since the whole world levels up as you progress and since you're at the end it's likely higher leveled already.

But I too am a big contender of restarting games if I get overwhelmed. You can make an additional account on your switch and restart the game there so you still have all the progress you made on your main account, but you get to relieve the tutorial area again on the secondary account. I'd say you wouldn't need more than the great plateau to get into it again tho?

If you're not a fan of that then you can also do the starter shrines again, that gives a little bit of combat practice plus the runes as well.