This time the system lasted almost 14 days before the lockup.  See the latest
attachment for the log details which contain NMI messages followed by a burst
of ascii null characters (which in my experience can be due to different
threads or processes trying to write to the same file, i.e., the NMI error
messages themselves might have exposed another kernel bug).  Unlike the last
case of NMI mesages where an Intel network card was mentioned, the only
hardware I can see
mentioned in these messages is a particular cpu and my motherboard, e.g.,
Oct 17 13:25:02 merlin kernel: [1177237.021995] NMI watchdog: Watchdog detected
hard LOCKUP on cpu 13
[...]
Oct 17 13:25:02 merlin kernel: [1177237.022042] Hardware name: System
manufacturer System Product Name/PRIME B350-PLUS, BIOS 3803 01/22/2018

So this appears not to be hard evidence of a graphics stack bug since likely
any linux system component bug could lock up a cpu, but I am still pretty sure
this is a graphics stack issue with the RX 550 because of my prior evidence
showing
much better kernel stability if I do not use that RX550 card at all.

I started a new up-time experiment using today's snapshot of Debian Buster
which left most of the graphics stack the same other than libdrm-amdgpu1 which
has been updated from 2.4.94-1 to 2.4.95-1 and the 
linux kernel which has been updated from 4.18.6-1 to 4.18.10-2.