I was in a very similar situation recently, having taken a few months off due to WaniKani burnout, and when I restarted, I not know the contents of the recent levels any more.
I was level 25 at the time, and my strategy was to use the Ultimate Reorder script to work my way up in my reviews. So I set myself an initial level limit (don’t remember exactly, maybe level 10, but please pick whatever makes sense to you), and I did all the reviews up until that level. I stayed at this level for a few days and only cleared the reviews of those levels, ignoring the higher level reviews. Then I slowly worked my way upwards until level 20.
When I tried to jump to level 21, I felt that I did not know the kanji any more and that the vocabulary would be overwhelming, so after a day of hesitation or so, I reset to level 21. In hindsight this was the best thing I could do at that point. I am now working my way up again, and it just feels good.
I think in your case, you might also be able to find out your “sweet spot” level for resetting by checking the kanji and vocab reviews level-by-level?