Level 23
In the end, I chose
  • very simple for a beginner
  • mimics macOS user interface - which is something they are used to
  • easy to install and update apps
  • Firefox was available - installed uBlock Origin and Emsisoft
After about 6 hours of use and tests, I think it will run well for watching livestreams and YouTube.
It's one of the (if not the) most beautiful Linux distributions out there.
Great new life for an old all-in-one pc.