There are number of threads around about problems installing on and booting from external drives. The general comment is that "Reduced Security" is required in the Startup Security Utility. My own successful test installs have all been done with Reduced Security.
But I have just installed onto and booted a Samsung T5 connected directly to my M1 MBA with Full Security set, using an 11.1 bootable USB.
But I have just installed onto and booted a Samsung T5 connected directly to my M1 MBA with Full Security set, using an 11.1 bootable USB.