iMovie library

> [...] I have seen some dated posts similar to this, [...]

   I haven't.  Thanks for the helpful links.  Were any of those "some
dated posts" in some appropriate (ReadyNAS) forum?

      https://community.netgear.com/t5/x/ct-p/readynas

> [...] According to mac, I should be able to [...]

   Where, exactly, did you find that nugget?

> [...] The format of the selected hard disk or network volume is not
> supported. Select a different location.

   And the "format of the selected hard disk or network volume" is
_what_, exactly?  Do we know which file system(s) iMovie supports?

   Or the file-sharing protocols which are supported by your NAS?

   "External storage device" and "Network Attached Storage" are spelled
differently for a reason.

      https://support.apple.com/en-us/HT203049

      You can store your library on an external storage device such as a
      USB Thunderbolt drive formatted as APFS or Mac OS Extended
      (Journaled).  Apple doesn't recommend storing iMovie libraries on
      external storage devices like SD cards and USB flash drives, or
      drives shared on a network.

   I do nothing significant with iMovie, so I know nothing, including
exactly how iMovie access its library, and just how much you might be
able to get away with when network access is required to use it.

   I suspect that you're doomed, but, If I had some time to kill, and
"Apple doesn't recommend" didn't stop me, then I might try to use a Mac
to create a pseudo-disk ("sparse bundle") on my NAS, use Disk Utility
(or something) to create a Mac-like file system on it ("APFS or Mac OS
Extended (Journaled)"), and see if I could get something like that to
work, but my expectations would be low.  (Too many layers of software
involved.)  But what do I know?