I had the same idea and after struggling to make it work, I did more (some) research and discovered that according to Apple you shouldn’t store Photos library on a NAS. It needs to be on a direct (not networked) attached drive/locally mounted volume. There is a complex workaround using a sparse image and APFS, and the instructions are available through a web search.(not sure I can link here). I tried it on my uGreen NAS but it seemed too risky and complicated so I changed back to using a directly attached RAID.