The preloaded spyware OS

Nowhere in that video did it say this. I am all for DIY NAS and I have an Arch-based one at home, but saying this while implying that that's what the source video you linked said is a bit disingenuous.

To be honest, nothing about this UGREEN is any different from any of the other off-the-shelf NAS solutions out there like QNAP, Synology, etc. If you don't trust the UGREEN pre-installed OS, you shouldn't trust any of the other ones either. I am not saying you should, but my point is that this pretty par for the course as far as pre-built NASes go.

Most companies do not provide support if you install a custom OS. That isn't a sign of vendor lock-in, just a matter of keeping support feasible in the long-term, especially since they're relatively new at this. If you want a custom OS, it is far easier and cheaper to just build your own.