Hello, sorry it took so long to get back to you. I have written up my explorations in markdown format, uploaded to two places in case one does not work. Mostly this is me going off on a wild goose chase about wayland input handling, and failing to build OurPaint aha :).
https://rentry.co/4yov76r4
https://gist.github.com/qwitwa/f6e2b9ffaaf64058c89b40a6df9b0fe5
If you are short on time, my conclusion is that filtering for the string "stylus" will give you the correct device.