Last night I took the painful step of just blowing up the networks altogether. I deleted the network.cfg and the local-kv.db for Docker, then set up eth0, updated to 6.12.10, added the other interfaces and Docker networks back, and then set up my dozens of containers all over again.
Three hours later I'm up on a current working state on 6.12.10 - still running macvlan as is my preference. I just hope that I get more stability on 6.12 this time than I ever did when I could update through 6.12.5. I'llmark this as the solution, but note that there are others who have had other successful workarounds.