Due to the original developer abandoning the project and it then promptly breaking, someone on Reddit fixed it.
https://www.reddit.com/r/redditprotools/comments/pkysl6/how_to_get_the_latest_version_working/
I've incorporated their fixes and published a CRX file for folks that understandably don't want to just use an unpacked extension.
This is not a promise of future support: I'm a back-end server development with JS experience rather than a front-end web developer. If it breaks for me I'll attempt to fix it, and when I do so I'll update here.