I wish LineageOS would support more tablets. I’ve got it running on an old OnePlus 6 I use as a Plex client and not much more and it just works, and I get all of the latest security updates etc. I know there are other ROMs out there, but none of the big ones support semi-modern tablets.
This is 100% unofficial. Phone makers don’t want you to install third party roms (they cease having any control, won’t receive analytics, can’t push ads etc). Why should they help you out? They’ll do the bare minimum (or less) that is required for you, the owner, to be able to use third party roms.
Put simply, you don’t. If your phone is supported and the documentation is there, the installation is a breeze.
If only more people realised this or could spare the 30 minutes it takes to educate themselves. I recently replaced the battery and charging port on an “old” Samsung phone (three years or so) and it’s as good as new. While I did use my 3D printer to soften the glue, I’ve been able to do the same with a hairdryer in the past. All you need is a couple of guitar picks or a set you can buy for next to nothing.