How can I isolate what is causing the temporary "stall" during startup?
I'm pretty sure it's disk encryption (FileVault2) that is causing the delay. I had to erase my internal flash drive and restore from backup. In so doing, I reformatted the flash drive as normal APFS and not APFS encrypted. There was no delay in booting. After I enabled FileVault encryption (and it finished), the boot delay returned.