Solved.
I googled a bit for raspberry pi stuck midi (or some variation of that), and found much discussion about Rpi unusual USB ports, and various problems encountered.
Found a suggestion to add this to the Linux kernel command line (/boot/cmdline.txt):
dwc_otg.speed=1
This limits the speed of the USB port to full speed (12Mbits).
I just played several different engines, etc, for maybe 1/2 hour, without any stuck notes, and with completely acceptable latency, even though I’m using USB audio.
This is very encouraging.