A big hurdle to this is hardware vendors locking bootloaders and making it impossible (or impractical) to write or use existing drivers.

Manufacturers maintain long running forks of Android (often very old Linux kernels) with their drivers hidden in their fork's source.

I'm a firm believer in the right to repair software - and the fact that it's illegal to reverse engineer binary blob drivers (or proprietary software at all) is a shame (not that you could even untangle a driver from a binary blob of a Linux fork). I'd go as far as feeling strongly that drivers should be open source, and if they aren't, documentation sufficient for the community to write drivers should be made available by manufacturers.

Linux on M5? Should be easy

Linux on an X Elite Surface Book? Should be easy

Ubuntu Touch on my Pixel 9? Should be easy

Android TV on my TV? Should be easy

Proxmox on my 5g mobile router? Should be easy

No drivers / locked bootloaders = not possible

>the fact that it's illegal to reverse engineer binary blob drivers (or proprietary software at all) is a shame

Where? I don't think it's illegal in the US at least. The only things I'm aware of that may have legal issues are related to radios, specifically modem/baseband stuff, and maybe WLAN cards.