Assuming the T2 chip allows you to run the OS from an external drive.(Edit), Scratch that, there seems to be ways around the T2 if needed for this https://www.imore.com/no-apples-not-locking-you-out-linux-macs-t2-chip.
Someone on the iFixit mac mini teardown site claimed you might not be able to boot the machine in the event the flash drive failed. Could you still cmd R into the recovery screen if that happened to change the security settings to allow external boot? He claims you couldn't, that it would require a logic board replacement at that point. The procedure in the imore article assumes the flash is still working, right. Hard to believe Apple would allow the failed flash drive scenario to disable the machine, though. Would like to put that to rest before I bought the mini.