Separate directories for system and vendor drivers. System directory given lowest priority.

Wouldn't require modifying vendor drivers or legacy installation or whatever. Just move the system drivers.

I must be missing something pretty fundamental here, because this seems like the immediate natural solution.

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Disclaimer: I've had basically no meaningful-to-me interactions with drivers that weren't mediated by an installer or package manager, and I haven't used any form of Windows in 10 years.