Burn-out is definitely something I have to take care to avoid, having burnt out hard on more than a few occasions in my life previously. There were a couple of times on WaniKani where I was facing potential burn-out, and in each case it was from going too fast, and the solution was to take a break from WK and then slow down my pace when I came back.
The first time, I was able to slog my way through the huge pile of reviews that had built up during my break (this build-up can be avoided by using Vacation Mode, apparently, but I never used that; don’t really know why not). Slogging through was tough going, but I did manage to do it.
Then around the time of Covid I was getting stressed out and needed to take another break for my sanity. By the time I came back, I had a huge mountain of reviews and was in no mood to slog slog slog again. I solved the review pile-up this time by resetting, and wrote a little post about how I came to that decision, and how it felt (good!), to hopefully help others facing the same/similar decision: After a long absence, I reset (to level 7), and I’m glad I did
At that point, I realized that even after slowing down the first time I took a break, I was still going too fast, i.e. my pace was ‘unsustainable’, which I think is a good word for thinking about it. So, I decided to really slow right right down, because it was important to me that I be able to enjoy learning Japanese over the long run, so that I could keep going at it steadily and indefinitely without needing to take mental-health breaks. Because, ultimately, my goal is to actually learn Japanese – not just to rush to get to ‘Level 60’, which was really just an artificial goal I had been imposing on myself.
At that time I came across this post, which is oriented towards people like me who want to take a ‘slow and steady wins the race’ pace, and enjoy the journey along the way: 
Let’s Durtle the Scenic Route 
. I periodically check in on that thread, and sometimes post little tidbits of ideas about how I personally have adapted the way I use WK to be more sustainable and enjoyable. If you’d like to avoid burn-out, it’s probably worth checking out. 
I’ve been Durtling along since then, smelling the flowers and enjoying the scenery along the way
. . .
… and Burn Out has left the chat! Said “brb” but I haven’t heard from it since! 