well, i've decided to start building it out, and i have the first 2 features i want completed for lists of games. i'll be working on a floating panel design for game pages and collection management. i welcome anyone who would like to join in! i am even willing to teach a bit.
the code can be found here: https://github.com/ericchase/browseruserscript--itch.io
you can test the userscript by installing "io.itch; add favorite,hide buttons.user.js" via ViolentMonkey on Firefox from https://ericchase.github.io/browseruserscript--itch.io/out/. more instructions can be found in the repo readme.
if i make significant progress on this project, i'll eventually turn it into a full browser extension