I'm not sure what your "theories" would be here, but boot issues are, as of Big Sur, entirely in Apple's court - no 3rd parties can copy the OS, nor can they affect boot. There could be hardware issues, software issues, etc.
One thing to try, just to see if it works in a basic sense, is to install the OS on the drive on its own. If that boots, then a copy should boot as well. If not, well...
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--Dave Nanian
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