Hello my fellow WaniKani users, i am stuck on level 40, after a long break i didnt use the time away function on the app and the reviews piled up into the thousands. Does anyone have any advice on how to get back on track and cut through the huge review backlog?
Be glad to hear what helped you face a similar issue!
I have a similar issue (but level 20, so not as severe). What I do is use the Self Study Quiz to refresh the WK items level by level. And when I get far enough with that I restart the WK review queue, sorted with lower levels first. When I start encountering stuff from higher levels, go back to more Self Study Quiz work. Rinse repeat until the backlog is gone.
Easy process, but a lot of work.
….you need to summon some of the gods for this (level 60) 
but i can say just go on vacation mode and try to clear them as much as you can
Having worked through smaller backlogs, I found setting the apprentice items first helped a lot. The only other thing that comes to mind is just sitting and working it. Momentum helps a lot. Try to find some time to just hammer at it. I found that even doing a handful a day, and then one day just devoting an hour to work on it, made my piles more manageable.
My first big pile as around 800 after a 10 year gap.
Just do a fixed amount of reviews each day. It will clear out over time.
200 or 300 reviews a day or so split into 2 sessions