I’ve not read the full discussion but I can comment on this.
openhabian-config option 2 will perform a full upgrade of all the software on the system but it does not move to the next distro. So if you are on Raspbian stretch and execute that option (which runs sudo apt-get update; sudo apt-get dist-upgrade) it will upgrade to the latest versions of all software for Raspbian stretch.
Buster is not strictly for Raspberry Pi 4s. The RPi 4 will only run buster. But all previous RPi versions can also run buster. I’ve upgraded my RPi 1s, RPi 0Ws, and RPi 3s to buster.
To upgrade to a new Linux distro is a lot riskier and error prone so I suggest a fresh install and rebuild of the SD card instead of an in place upgrade. If you want to take the risk you can try an inplace upgrade. In my experience it worked three times out of five.
grep -rl stretch /etc/apt/ | sudo xargs sed -i 's/stretch/buster/g'
sudo apt update
sudo apt dist-upgrade
It takes a couple of hours on the RPi 0Ws and RPi 1s.
I think there is work going on to make buster the base version when you install openHABian with the SD card image.