If you’re feeling iffy about WaniKani, then “read this few-thousand-word guide!” probably isn’t what you want to hear, but at least flicking through this might give you some insight that will be useful:
For what it’s worth, I third the recommendation to use the double-check script. It’s an absolute lifesaver for typos. Also, I consider myself a fast WKer (but definitely not a speedrunner) and I level up around every 10-11 days. That’s just how long it takes.