I personally use a modified version of this /t/WaniKani-Rainmeter-Windows/1108/1
The modifications I added are just a huge lit indicator that changes color depending on how many reviews I have. I’ve also set it up to start flashing red if I have too many (I also have a bar under my taskbar that will also flash red, thanks to transparency it will show up even if I can’t see my desktop). Basically the modifications are just to annoy me until I do it … I’ve not needed that yet though