I agree with @DerRittmeister and @Pluckabee that you’re probably best off limiting how fast you do lessons. I tend to try to level up quickly, so here’s how I do that while keeping things from piling up too much. Note that this is only really possible since Wanikani started giving you lessons in level order. I guess I could have used a script before, but I never tried it.
- When I level up, as soon as I’m able, I do the lessons for the remaining vocab from the last level and the lessons for the new radicals, then I stop.
- I do reviews only for a day or a day and a half, then do the lessons for the first kanji in the new level.
- After that’s sat for another day or so, I’ll do all the remaining lessons, which are just the first vocab for the new level.
- At this point, the radicals and kanji will start to move up to guru in a naturally staggered sort of way, and I’ll do the resulting new lessons as they come up.
This method doesn’t reduce your average review load over, say, a week, but it does help to spread reviews out more evenly, so you don’t have a ton at once and then nothing for a while. I also expect this method will stop working in the late 40’s, but I’ll deal with that when I get there. 



But that’s what makes wanikani perfect xD Usually reviews pop up at 11am haha and that’s the peak time to sleep and I’m doing my best to finish the reviews omg xD You just really need to finish all of your reviews before you sleep :V