Thank you for your input. 8.90.003 is not running much smoother. The long pauses have stopped on most but not all videos and now I have micro stutters because frame rate adjustment does not seem to work (yet?) in Kodi 18. Video output is always 1080p60 no matter the source frame rate.
I have to admit I'm a total Linux noob and have no idea how to get the logfiles from that device in here. I don't think I want to spent countless hours now on learning how to do this and searching for the source of the problem.
The little spare time I have is too precious to me to waste it on stuff like that.
So I've decided to send this unit back and get the very same 3B I already have. Again, thanks for your suggestion.