In case it’s useful to anybody else, I tried @Fernando_Pereira’s earlier suggestion to:
apt remove alsa-utils
That worked, no more amixer trouble on reboot and Roon Bridge works perfectly. It would be great if Roon Bridge did not bring in all that unnecessary ALSA utility cruft.
It didn’t work in my case. USBridge disappeared from Roon (perhaps because I have no other audio applications in DietPi, except Roon). I then did:
dietpi-software reinstall 121
and all is well.