This adds some risk of skipping vocab temporarily, but because you mentioned doing all n-2 items first I guess you can never more than one level behind on vocab. I wonder if this would only keep you at optimal level up time for a single level though, and then you’ll be back to catching up on vocab. 
Personally I use [Userscript] WaniKani Lesson Filter, so I wouldn’t need this. But it’s not a bad idea.