Things like these are still the reason I am not switching away from Mac OS
Am writing this from such a setup on my TP.
UEFI is fine booting a third party OS when SecureBoot is disabled BUT the T2 chip blocks accessing the internal storage from those OSs [1][2]
[1] https://www.reddit.com/r/linux/comments/a4thsc/the_actual_fu...
[2] https://twitter.com/LinusTech/status/1067232394934083584