Congrats on level 20!
Or if you’re more of a dog person, わんわん探偵団. I couldn’t find the USA Amazon page for this one, but maybe someone else has it (“Bitch Detectives” doesn’t work).
The grammar might be a bit higher than what you’ve studied, but you could check out the reading group right here on WK for volume one of the kitty series. We finished up a while ago so there are lots of questions answered for the entire book. There’s also this thread someone started because they felt their grammar was too low and everyone talked about whether it’s worth it to get the book at whatever level they are. You could read that and decide before you buy.
My personal experience (I read the kitty one, volume 1. Working on volume 2, might do the doggie series next):
Vocab: This is my 2nd time through WK after hitting level 60, and there were still a ton of words I didn’t know (70+ from the whole book). So vocab wise you’ll be seeing a bunch of new stuff anyway, I don’t see a reason to wait to advance in WK for this particular book.
Grammar: Finished Genki I and II, and now in Tobira (at the time just getting started, around Chapter 1 or 2). I didn’t feel there was anything too tough grammar-wise. In one of the kitty threads I remember seeing a comment saying that some of the grammar was around N3 level and if they had known that before starting, it would’ve put them off reading it. But they read it anyway and were pleasantly surprised at how they kept up.
I hope that helps!