You might get, as @UInt2048 suggest, more value out of learning additional vocab rather than pre-learning kanji.
May I recommend [Vocab] WaniKani Expansion Pack v1.0 - 1/13/2016 - Now on Memrise! which will teach you 4000ish bits of common vocab that wanikani doesn’t teach, organised by WaniKani level. I have a version of that deck in Kitsun.io and it’s been a great way to expose myself to common grammar that would be useful to know, but wanikani can’t fit in.
The example given of 下り is a good one for the kind of vocab you’d learn. It means downhill, for example, which you’ll agree is common and useful.