You should be able to turn off the T2 function Secure Boot which blocks booting from other drives: https://support.apple.com/en-us/HT208330
According to this newer Linus Video [1], one of the T2 SSD slots still needs to be populated with an Apple drive to boot from other drives.
Which makes sense: it needs to load firmware/EFI from somewhere; and in a setup where you expect your own SSD to be a fixed resource in the system, there's little reason to have separate BIOS flash.
[1] https://youtu.be/mIB389tqzCI?t=180