viking3r's post in Odroid Hc1 Docker & Wireguard was marked as the answer
So the prior replacement of the linux-image seems to be the culprit
Makes sense in hindsight
I now installed wireguard via
sudo apt install -t buster-backports wireguard wireguard-dkms wireguard-tools linux-headers-legacy-odroidxu4
now it works
I rebooted a couple of times no issues there.
So in the end it seems like it was the install of wireguard who led to all of this.
The one time I could reproduce the device not rebooting without having it installed must be something like tparsys said.
So for now the problem is solved. And some knowledge was gained
I should pay more attention when apt wants to install some packages.
