Catalina photos library - crashes, goes slow, presents fuzzy images

trying not to rant....and a little bit of history

MacBook Pro 2018 - 25000 photos and 1500 videos

Mojave - all photos and library off-mac on a Synology NAS drive ,all working fine, could import with no problem, photos was responsive, finding faces seemed a breeze, all in all a happy situation.

Catalina - started out with photos and library on Synology NAS as before, but nightmare - had to rebuild library at least twice, and could not import any new photos. Was told by phone support it was due to fact that library off-mac on non-mac format disk (a no-no).

So copied 64GB library to local disk. But, still had to rebuild at least once. Photos is totally flaky, often photos are blurred when I start it up ( I'm not syncing to iCloud) sometimes not, It goes off for a think for 10 minutes, then stops working, crash report etc etc. Had to hard kill at least 5 times. And I've not even mentioned the progress of the scanning, yet.

What are the parameters and constraints within which Photos actually works ? Can anyone tell me that ? and when will it finish 'scanning the remaining 25343 photos' that it keeps telling me its doing, but numbers are not changing.

Please point me at some sites/docs/posts that tell me:

Where do all my photos have to be located for photos to work ?

How do I make sure that 'scanning the remaining X' photos actually finishes ?

When will someone contact me about all the crash reports I keep sending in ?

TIA

[Edited by Moderator]