I also took a close to a year off and then came back to Wanikani, also at about level 20. I never really considered going back to an earlier level, but just pushed on through the review mountain. I don’t regret it, although I suppose I can’t be sure that I would regret it if I reset either. I seem to have retained a good deal, and they have chosen the kanji vocab on this site pretty well so you will still come across kanji you earlier learnt in later levels in words where they are combined with kanji you recently learnt.
In short I would say you could probably individually just revise the items that you’ve previously burnt but which you are not too confident on. If you haven’t burnt many items, by the time you work through your review stack the kanji have forgotten will have reset to the guru level and then its just having to put up with a pretty high review stack for a while (easily doable as long as you don’t push it and do more lessons at the same time).
Proof of my earlier unhealthy studying habits (I’ve gotten better):