NAS Drive and Photos Library

So, I bought a SynologyNAS drive because my iMac was getting quite full with the intention of storing both my iTunes and Photos libraries.

iTunes presented no issues whatsoever, but Photos has.

I copied the file over to a folder on the NAS drive and opened Photos by clicking on the new file.

I can see the photos, but the small preview images take a long time to appear, and it was this issue that caused me to research the internet to try and resolve it. But I've failed - so far!

• The lcation of the data file is shown correctly in Photos, but I am unable to "Use as System Photo Library" as the button is greyed out.
• The Sharing & Permissions section of the Get Info shows Read & Write settings for two (unknown) and one (everyone) users. I've checked the original file and the permissions are correct.
• When I try to add more users I either don't have permission or the name of the user can't be found.
• I can't delete the existing user permissions (not that I would) because they're greyed out, too.
• I tried to repairing the library, but this fails around 3-4% complete with a permissions error.

Anyone got an idea on how I can resolve this, please?

Thank you.