It’d be hard to give specific reading recommendations for each level for a few reasons (but pretty much all of them are centered around the fact that WK is a kanji-reading site and not a general Japanese-learning site):
We don’t know how much you’ve been learning or practicing grammar outside of WK, which is a large factor in determining how readable a text will be. (Possibly moreso than kanji, since you can’t just look grammar constructions up on the fly–even though obviously too many unknown kanji are also a deterrent.)
WK isn’t structured around the frequency of its vocabulary, but the difficulty of its kanji, unlike, say, a specifically vocab-focused SRS app (like iKnow and others) might be. This means that there’s a mix of obscure and foundational vocab spread out over all its levels. It’s not a simple step up in complexity each part of the way. (It is for its kanji, but not necessarily the vocab tied to them.)
That said, I’d totally suggest perusing reading material on your own–whether that means graded learners, easy news apps, light novels, or diving in on an even deeper end. I’ll say that in the 30s, kanji should no longer be the biggest hurdle for you in reading. That isn’t to say you’ll be able to comfortably read all adult-level prose in the 30s–it’s just to say that kanji should no longer be the most significant hurdle. (There will be unknown ones, but even most adult-oriented books will still be delivering them at a comfortable n+1 level where the unknowns are less common than the knowns, so you can start to remember them on your own.) So if your grammar and general vocab are sufficient, you can start making your way through. (You can also start making your way through even earlier.)
Also though, just, like, go ahead and try stuff that’s too hard for you. Worst-case scenario, you decide it’s taking too long because you don’t know enough vocab/kanji/gramamar yet (so it’s “too hard” for your current level), and … no loss, you know? You probably learned something and can better gauge your next attempt. There’s no science to this, so get reading.
Edit – But also, after level 1? Yeah; probably graded readers, though you should also just be learning foundational grammar in some way, I guess. For every level under 10, if you’re also just starting with Japanese, most native material is going to provide too big a hurdle in terms of both kanji and vocab to really be useful. I think the most I could say is that I feel like prose without a lot of furigana attached will first start to feel really accessible somewhere around the 20s. And by the 30s–again, if you’ve also been steadily learning grammar and outside vocab–basically anything but extremely complex native prose should feel accessible even if you have to grind through a lot of unknown vocab and lack of comfort with reading native prose in the beginning. But even before that, you can probably tackle younger-skewing light novels that are gentler with all three of word count, kanji and vocab.
Edit edit – I’m actually kind of a bad person to ask about this, now that I think about it, since I read manga in Japanese while below N3 level, and then around N3 level just jumped straight into adult literary prose and ground through it until it started feeling easier. So I skipped graded readers, light novels, etc. But I’m here to say you can do that. All of this was painfully slow at first, full of look-ups and settling for half-understandings, and then slowly that gave way to quicker reading times, fewer look-ups, and full understandings. Language knowledge is one thing, but reading is also just its own skill you have to practice (ideally while taking notes on vocab and phrases to whittle down the number of unknown elements as you continue).