The easiest way to reinstall
iOS Big SurmacOS on M1 Macs is to treat them as what they really are, glorified iOS devices, and DFU restore the operating system from another Mac with Apple Configurator or similar tools (macOS is downloaded as an .ipsw file and deployed on the M1 Mac). Works like a charm and is quite fast and guaranteed (barring bootrom/physical hacks) to ensure the device is not running non-Apple software.
P.S. T2 Macs are actually the most difficult to restore to the latest OS sometimes. The installer is anal about asking local password, booting from external USB, etc. and you might end up having to go through internet restore once and then upgrade the OS. M1 is actually an improvement and simplification from clean OS install perspective.