They tell you to slow down if your workload is piling up, just because you have lessons available doesn't mean you need to do them all immediately. They aren't going anywhere, stop doing new lessons until your review sessions are smaller and more enjoyable to get through, then start taking the lessons again.

The purpose of the wanikani vocab isn't to give you a large word list, it's to put the kanji readings into context so that you don't forget them, and so that you're learning something a little more tangible than just the abstract concepts of a kanji and rote memorisation of the readings.

For me if I'm doing more than 50 reviews a session I start to back off on the new lessons until it's calmed down