I did some additional tests.
Before rebooting IPAD, IOS tries 2 times to reach thermalmonitord service, every 60 seconds.
It then creates 2 Stack+thermalmonitord logs and then it crashes creating the panic-full log the third time
In my case sensor TG0B is not reachable. This morning I noticed that the charging connector was really dirty so i cleaned it up with a toothpick. After that I tested ipad for hours and i didn't have a single issue for a first time since weeks
Sadly, this evening it started againg to reboot randomly (same problem).
My opinion is that there is something on flex cable that is wrong. Cleaning or replacing it will probably solve the problem. The spare part itself is really cheap but it's not easy to replace with a new one (soldering skills are needed), and apple will not do this for you.
Probably with new IOS versions they changed the way how thermalmonitord watchdog handles the system. Is possible that we had the stack+thermlamonitord logs even before, but they did not cause the reboot.