I didn't read all of the thread, but has anyone suggested and/or have you considered the dosdude1 Catalina or Mojave Patcher? I upgraded my (oficially unsupported) late 2009 iMac to Catalina and it is working fine. YMMV, of course, but ijust make sure you have a backup.
Each year that Apple releases a major new version of macOS, a few older Mac models that ran the previous version usually fail to make the new list of...
www.macrumors.com
There are "how to" videos available on his website, if I remember.
Edit: I just saw someone else recommended this as well.