Updating this. Got pulled off this project and then put back on.
I’ve done some investigation, and saw that the serial-getty@.service times out 8 times, and then gives up, whereupon the rest of the system can boot. After that happens, serial-getty manages to successfully initialize, and the problem doesn’t come back.
After playing around with my hooks, actually found an issue that has somewhat similar symptoms: Snapd.service delays startup in Ubuntu 18.04 with 4.15.0-24
I was using 4.14, so I tried upgrading to 4.19 to see if that fixes anything, but that had no effect, so the problem must be elsewhere.
Also, it only takes about 10 minutes for the serial interface to give up.