The issue happened again, and each time it occurred so far, it was after some heavy write activity over several hours which resulted in the cooler becoming noticeably warm (but could still be touched without burning the hand). Could heat be related to this? Its strange that it would only affect the bootloader though. Any ideas what heavy load could have to to with this otherwise?

Ah gotcha.

We do have 3A https://fi.mirror.armbian.de/incoming/igorpecovnik/rock-3a/archive/ (need report if this latest build works) and build config for 3C but not 3B. Does it make sense to make for 3C too or 3A can cover them all. I don't have any of those, so I can't check.

He probably meant upstream driver version v5.7.9_35795.20191128 https://github.com/jwrdegoede/rtl8189ES_linux/blob/rtl8189fs/include/rtw_version.h and by looking into Makefile it feels as we are in 2010

That isn't as simple of a question as you may think it is. It depends. But since you are dealing with a TV box, that would be better asked in a question over in the TV box forums as it is off topic for this thread.