Regarding the SMB-share, let my try to clarify. Let’s say you have 3 machines. 192.168.1.10/20/30. On machine 10 a folder synology which has a network folder mounted onto it from machine 20 mount -t nfs 192.168.1.20:/some/folder synology.
Now you want to access that folder on machine 30. Here you can’t use mount -t nfs but MUST use mount -t cifs instead, because you cannot forward a mounted share. However, this is not the problem, it’s just a description of my current setup.
Regarding the ownership. Your point is very valid, but I ruled that out already. I did a so-called bind-mount within Synology with the exact user permissions as in the users home folder, but this didn’t work. FYI: a bind-mount is where you have two folders /foo (with many sub-folders and files) and /bar (empty). If you do mount --bind /foo /bar, then the system thinks that bar is a real folder with the subfolders and files (from foo, including their permissions).