I burnt out the same way a few years back doing WaniKani. I was initially doing as much lessons as I could for the first ten levels then transitioned to 20 per day til I burnt out.
On top of this, the biggest mistake I made was not immersing in Japanese i.e. reading. This convinced my brain that I only needed Japanese for WaniKani and that really brought my retention down to the point where I burnt out because I kept getting my reviews wrong and could never remember my lessons even if I did only 5.
I was stuck on level 31 for 516 days and 29 and 30 for 100 days each. I made it a point to still do reviews but I never did any lessons because I felt that I couldn’t remember any more words.

On a whim, I started playing SMT IV on my 3DS along with Yokai Watch 2 in Japanese just to see if I could do something else to “restart” from my burnout. To my surprise, I was able to actually play (with very generous dictionary usage) and comprehend most of the stuff in game. But my biggest rule here to myself was to just do 10 mins a day. No more no less. This took much of the weight/pressure off having to really be in that study mode because it was so short. And this proved to my brain that I could still learn even after burning out.
This in turn re-motivated me to tackle WaniKani in a more sustainable manner. If I did only 5 lessons a day during my burn out I would be level 60 by now. So with that in mind I went back to it starting at 5 lessons a day and worked my way up to 15.
This also happened to coincide with their own implementation of locking lessons to a set amount per day and to hide the total amount of lessons available. This was perfect for me because I always felt way too intimidated every level up to see another 100+ lessons available. The sense of accomplishment when you zeroed out your lessons never happened anymore because I couldn’t keep up. But with the new update, every day I get that sense of accomplishment that I’ve cleared out my lessons for the day.
I’m finding that 10 lessons a day is a sustainable thing to do everyday. I personally do 15 right now but that’s because I have more time in my day to spend for reviews. With 15 lessons a day for me and with reviewing at the start and end of my day, I average around 11 days per level. However I average around 100 reviews for my start of the day review and about 80 for the end of day review.
So now I’m on track with my pace to hit level 60 by the end of the year (hopefully).
Surprisingly, the thing that keeps my retention locked in for WaniKani isn’t WaniKani, it’s actually my daily immersion of reading and listening to Japanese. This tells my brain that the Japanese language is a priority and (probably) helps retention because my brain now allocates more resources towards retaining the words I learn in WaniKani.
It’s been said often that WaniKani is a supplement and not the main thing, I know that this is common knowledge amongst learners but I think because WaniKani is a very long journey, we tend to forget that there’s a whole world of Japanese outside of WaniKani. And with the amount of mental power WaniKani asks of you day in, day out, we tend to not want to interact with Japanese after WaniKani because of how mentally draining it is.
However, as the saying goes: “Use it or lose it”
And this means using it in the real world away from WaniKani. Even if I check the dictionary every other kanji, the effort I put in shows my brain that this is something that needs priority and therefore needs to be remembered outside of just WaniKani.
So personally the main thing that got me out of my burn out was the same reason I got started with Japanese: To be able to consume and enjoy their media at a native level. However the thing that keeps me going day to day sustainably is limiting my lessons to 15 a day and just doing it everyday whether I feel like I can remember or not simply because the SRS system will handle the remembering for my anyway.
EDIT: added a pic of my burnout and my “rebirth” Just to really illustrate my point.