After some more investigation I'm suspecting that this may have been
unrelated
to the fw update. I've seen 2 times since where my phone has
failed trying to connect to the 5G until I rebooted the router. I did not
think the boot delay and message was caused by this because I did not think
the system connected to wifi at boot time, but now I think I was mistaken.
I see on the login screen that the wifi is already connected.
I will say that attempting fw downgrade is probably dangerous. That's
probably true of all software as upgrades are usually tested, but not
downgrades.
Thanks for all your help!