@curtisj - does this help you at all - it worked for me (twice recently) https://docs.netgate.com/pfsense/en/latest/solutions/sg-1100/reinstall-pfsense.html
I have around a dozen SG-1100 in production, they tend to arrive to me in two's or three's so I configure, and software upgrade them one after another, and ship them out. This gives me a some sort of impression of how consistently they behave. Anyway, the last three arrived with me just over a month ago, and I performed a vanilla out-of-the-box config on them, and then the upgrade to pFsense Plus. One decided to brick itself rather than take the upgrade (so I used the procedure above to recover it) and the other two took the upgrade just fine via the web ui. And now I'm thinking about it, there was one that I stuck a label-printed note onto the top that I was unable to serial console onto it.... Maybe I'll revisit that unit and use the warranty. As I'm supplying them to technically-capable users, but not neccessarily firewall technically familiar, I perform multiple reboots before shipping them out and always keep a copy of the final config. But again in this most recent batch one decided that it would be a brick after being DHL'd to the user. Used the procedure above, and blasted the config back onto it, and it's been fine since.