I’m seconding this!
If you’re impatient to learn new kanji, I recommend (once you get through your existing vocab build-up) using the lesson filter script to learn a few kanji a day alongside some vocab lessons instead of learning the kanji in huge bursts and then having to chew through day after day of just vocab lessons. I’ve found that spreading the kanji out actually allows me to learn the vocab lessons much sooner after guru-ing the associated kanji. Otherwise, I’ll guru a whole bunch of kanji at once and it will take me several days to reach some of the new lessons.
Some people are able to do all of the vocab lessons done before moving onto the next level, but I have seen far more people here burn themselves out doing that, because by the end of WK it can result in a hugely inconsistent workload, with hundreds and hundreds of reviews on some days, and very little on others.
If you are interested in going super fast, the method described in the ultimate guide to WK is probably the most efficient and sustainable.
But if you’re comfortable with 10-20 new lessons each day, it might be best to stick to that pace and level up slower than max speed. If you want to spread out the kanji, you could try doing 3 kanji + 9 vocab lessons a day (12 total), or 4 kanji + 12 vocab lessons (16 total). That should allow you to get all of the vocab lessons done shortly after their associated kanji each level, while also reducing the number of days you’ll spend getting through a vocab lesson backlog at the end of the level (you’ll only have a couple days of only vocab lessons at the end while you wait for the current level’s kanji to reach guru).
Regarding your 400 lesson backlog, I highly recommend pacing yourself and getting through them slowly, rather than trying to do 100 or multiple 100s in one day just to get through them. Many of the vocab lessons are going to be a huge struggle because it has been some time since you learned the kanji originally. If you’re not looking forward to getting through the backlog, you might also consider resetting back a couple levels and doing them properly.