Jim MacKenzie

There are only two different hardware layers. Stock kernel (4.4.x) and mainline (5.10.y)  based. Debian or Ubuntu is just application lawyer which doesn't have any effects on board hardware. Check modern kernel - it should be much more stable. 

Thanks, Igor.  What is the simplest way to shift to mainline?  (Preferably without reinstalling.)  i.e. Which linux-image package is the correct one for this platform?

I think this is related:
So it looks like one of a known ones. If we had more 
... this could be clear from quick forum scan.

If you mean that you don't see any kernel logs after "starting kernel", make sure that /boot/armbianEnv.txt have "console=serial" and "verbosity=7" ...