Hmm, no reply so far... so this is what I did. I saw that all the pics were in fact in the iCloud. So I decided to take the risk and delete all the pics and videos on the hard drive (I have a backup!) and then download them all from iCloud. This seems to have worked. Now all devices are in sync.
However, even now after a few weeks, none of my devices, iphone, ipad and Macbook Pro are able to browse all the pics. It's incredibly slow and very often I get the message : Cannot load Photo! Very annoying. Does anyone else get this? It seems the cloud cannot handle this mass of pictures. Surely I am no exception out there with this number of pictures...
I am very frustrated. I just can't find a good solution for my picture library. Photos seemed good, but it's not working smoothly enough. Am looking at Adobe, Amazon Cloud Drive, Flickr - but they all have certain deficiencies - iCloud would have it all - syncing, editing, storing, sharing, uploading RAW files... but it's the most expensive out there... and it's not really working...
Sigh. My Synology station is not doing it for me either. I suppose I am asking for too much.