As a quick major update to this, I have good news and bad news.
The good news is I have a better understanding of how things work now with Stimulus and I see that rfindley already mentioned some things in another thread that helped me figure out how to interact with things in a better way.
The bad news is that the script was relying on jStorage, which wanikani was using, and which provided access to current review items, but will no longer be used.
My goal with this project was to provide users of this script who were feeling a little more adventurous with a quick few steps to get the script working in the short-term. Unfortunately, this bad news means that even if I can get it to work, it won’t be easy for the average user of this user script to manually change it themselves. So at this point I’m not sure if I should continue to figure out a quick fix for myself or if I should potentially reach out to rfindley myself to see if I can contribute to the mainline fix.