The QNAP OS is already a variant of Linux, so with enough work you could make it run anything you want. I haven't tried or looked for instructions on it, though.
But, the data is more important than the OS here. And the filesystem is a variant of ext4 and there's some instructions floating around on how to mount drives on an external standard Linux machine to recover said data in the event of a disaster.
You should also back up the data regularly so it doesn't matter if the hardware or company fail.
Well the trick is, can a user do it themselves. Is there a locked bootloader? Proprietary binaries? Etc.
I'd be find with a vendor linux, but vendors tend to get bored with older products, drop support, go bankrupt, get purchased, revoke previously promised functionality, have security holes in their addons, etc.
But, the data is more important than the OS here. And the filesystem is a variant of ext4 and there's some instructions floating around on how to mount drives on an external standard Linux machine to recover said data in the event of a disaster.
You should also back up the data regularly so it doesn't matter if the hardware or company fail.