No, you’re not dumb lol. If you’re failing that means you’re learning, language learning isn’t a race. Keep in mind that most people don’t post their failures, so if you’re browsing forums or social media it’ll seem like everyone else is a genius and you’re struggling to keep up when in reality, your experience is actually pretty dang normal.
WaniKani isn’t the only way to learn Kanji, and you seem to know that since you’re learning Japanese outside of it. I hear most people who quit WaniKani do so when they hit around 20, so you’re not alone in thinking there’s other, better ways for your learning style. I myself have been thinking of quitting and just doing immersion and sentence mining with Anki, which I’m already doing concurrently right now anyway, though I think I might wait until I get to like level 30.
If you don’t want to quit, since I assume you got the lifetime sub, then I say just don’t sweat about it too much. Treat WaniKani like an auxiliary learning tool. Keep doing the reviews so that they don’t pile up, but maybe just like stop doing lessons, and even when you do do lessons, just do like 5 every now and then. idk if this is the type of response you’re looking for since it’s kinda quitter’s POV lol, but like overall I’d say don’t worry about it dude, you’re doing fine.