Still not working.
Put the working SIM card back into the old Samsung phone. Put the new SIM in the Pixel. Initiated the transfer process. Lost service on the old SIM. No change on the new phone.
Did a factor reset on Pixel. No change
Flashed stock Google Android (TP1a.220905.044) on Pixel. Still no change.
Moved new SIM into the old Samsung. Got cell connectivity after reboot.
So, it appears something seriously wrong happened in a low level component in the Pixel 5 when the OS was updated. Looking back, I rebooted my phone while in a location with bad cell coverage, so Im going to wildly speculate that after the OS update, the cell modem decided to do a firmware update or something and failed 1/2 way through do to bad cell signal... but that is just my wild guesses at this point.
So, to summarize: Im stuck. My Pixel 5a phone was running just fine until I rebooted to complete a system update. After then, the "No SIM Card" error will always display on my lock screen. Neither replacing SIM cards nor flashing to stock Android fixed the problem.
Any other suggestions for me to try before I go and order a new phone? :(