Fell off the wagon, how to restart?

Either you power through the reviews, or you reset to an earlier level.
I would suggest powering through the reviews, if you can find the motivation,
because resetting will, surprise, set you back ^^
People have defeated review stacks of 4000 before, in 2 weeks or so.
Consider that reviewing isn’t so different from doing lessons, you’re strengthening your knowledge of Kanji. And if you have trouble with certain items, just take a good long look at them and their mnemonics again, as if it was the lesson again. That’s what i do constantly.

You can also do separate training of leeches (items you have trouble with), so that they don’t come back so often, with the WaniKani Self-Study Quiz script and then getting the additional filters for it, which includes leech training. But that might mean more reviews of course, if you train more than leech reviews coming back.

Maybe try to listen to this for motivation:

(Recently featured in this video)

All the way!