Add the “Activate Linux” watermark as well.
The one from https://github.com/MrGlockenspiel/activate-linux

there is a bridge, but some parts of the fediverse got very mad about it
https://github.com/snarfed/bridgy-fed/
https://fed.brid.gy/

Other comments already pointed to some very good software solutions.
But I would argue that absolutely the biggest barrier to entry for the masses is hardware.
Restoring an old PC or making some cable spaghetti with some SBC is currently too advanced for average person.
Self-hosting for the masses would require some new form of home servers.
Something modular, where adding new components would be as easy as playing with Lego bricks.

GotHub seems to display basic GitHub stuff decently well.
https://gh.whateveritworks.org/

so this is similar to LibRedirect?
https://github.com/libredirect/browser_extension
https://libredirect.github.io/
check out this list of privacy frontends and see if you can implement any of them in Predirect

Also, I see that you only handle one instance for redirect. Either the default instance or custom instance.
Consider implementing multiple instances. Here is list of instances that LibRedirect uses:
https://raw.githubusercontent.com/libredirect/instances/main/data.json
Here is a list of instances that Farside.link uses:
https://raw.githubusercontent.com/benbusby/farside/master/services-full.json

Both LibRedirect and Farside still have to deal with the rate-limiting problem with Youtube and Reddit and I assume other sites as well. Invidious instance or Teddit instance can be rate-limited, resulting in video not playing or post not showing up.
If your extension does not distribute user traffic across multiple instances, then I assume that your chosen instances will get hammered into ratelimit even faster than other instances.
So consider thinking about solution for this.
One of suggestions that I like is to allow user to provide an URL to a list of instances.
That way, someone or some automated system can periodically compose a list of instances and users can “subscribe” to that list.

Also some useful links:
https://github.com/digitalblossom/alternative-frontends
https://github.com/pluja/awesome-privacy#social-networks-and-platforms
https://github.com/mendel5/alternative-front-ends
https://matrix.to/#/#alternative-frontends:tchncs.de