Report: I couldn't resist and bought a new unit in mid-December. I was not shocked when I got the exact same reboot behavior for the first call after pairing the Surface mouse. It was not fixed after a hard reset. OS was still .0 btw as there was no .29 available, which I got instantly with the first device. I was about to put it back into the packaging but tried again after another hard reset. This time it did not crash, but lost Bluetooth connectivity exactly after the amount of time when it would crash before. Even more to say, I did not experience any reboots since then! So I decided to give MS another chance to fix it during the next 2 weeks (return period). Next day I pulled the new .36 through the Insider program, but I did not notice any changes.
Then, after using WDRT, I installed firmware 15506 together with non-Insider .29. Bluetooth seems to be really stable now for me and the phone runs really fine. Only annoyance is the really short battery life compared to my 1520 (also W10M), probably caused by Edge.