I did eventually manage to set up some kind of chron job which wrote the log to a file every few seconds. I can't remember how sorry - it was more than two years ago.
The log didn't give me an answer, which I found puzzling. I wondered if the log was not being copied quick enough before the shutdown.
That might be an issue for some people, but I soon ...
Just create Magisk module and place your .ko kernel modules in
and create script
No need to modify /vendor partition