I've had several issues booting from an external for a number of years, so I don't do that now. Also, there's issues if you want to run stuff headless *with* encryption turned on. Combination of those two meant it became a PITA. What I do for machines with external primary storage and I want to run headless - I.e. reboot, connect remotely etc. without having to physically be in front is:
:- Configure MacOS on the internal drive for an Admin user, not signed into iCloud or anything, NO FileVault encryption.
:- Configure my main user, move its home directory to the external drive, turn on iCloud/FileVault etc.
Downside is that when I reboot I must login to the internal drive Admin user, then logout, backing as my main user. I only have to do that with a reboot and/or powerless. I've played around with mounting the external pre-login but had little success.
This is a bit of a faff but its been reliable for a number of years now, so much so I've not bothered looking for a better solution.