I now have a stable system (6 days uptime and counting) using a SanDisk Extreme PRO solid state flash drive as my boot drive. It survived the greatest load I’ll ever put on it during this time: transferring over 19TB of data to it from another NAS.
So the cause of my problems was using the UGREEN NVMe enclosure. I also tried a different NVMe enclosure (SABRENT), which was a much better physical design, but gave the same USB warnings during boot up as the UGREEN, so I assume would be equally unstable.
And the way to investigate failures of these sorts is to pay attention to boot up messages, and look at the local console by attaching a monitor if the system misbehaves.