I’ve been in this situation a couple of times now. I tend to use WaniKani Reorder Ultimate script to order by level. That way, you clear out all the older stuff first. You will probably burn a lot of those or get them to a higher srs level so they won’t come back again anytime soon, so your overall review count goes down and stays down.
Keep doing that and continue chipping away at the reviews. Eventually, you might get to a point where you are on reviews for a somewhat recent level. Either you will continue to cut down reviews and eventually clear your pile, or you will start to get a lot of items you can’t remember. In the latter situation, I’d try to evaluate your options. Either continue with a low accuracy and essentially relearn through reviews, reading the info when you get an item wrong, or reset your level so you start back at the level where reviews start getting problematic, and continue from there in the normal way.
I did this twice now. Resetting from level 25 back to level 22, and just last week from level 26 back to level 24. In both situations I just felt like it would be quicker to start progressing again if I just went back and went through the last few levels again rather than continually working on a large stack of reviews I have trouble remembering.
I end up in this situation because I tend to do most of my reviews at work, but when I am very busy in work I tend to not do any reviews and let them stack up.
TL;DR - WaniKani Reorder Ultimate to order reviews by level, just keep working through them until you start to get a high failure rate, or you feel the items are becoming a bit too unfamiliar, then evaluate whether it would be quicker to reset your account back a few levels or continue hammering away at the reviews.