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 officially supported Macs. In the case of Catalina, the latest macOS version, the operating system doesn't work on models released before mid-2012...
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.