If you only need to mount shares from the Synology box on the Ubuntu box, consider using NFS...that's a slide button and text box or two on Synology side and a /etc/fstab edit on the Ubuntu side. If you also need to share from the Synology to the Win11 tower, samba/smb may be easier out of the box, but you could simplify by adding NFS tools, which are buried in some windows feature add-on somewhere in the OS. Ive added this feature to mount shares but never to host them.

Hosting NFS from a non-Synology machine will be hard to explain here but a search for "setup NFS server" should get you on the right path. It may just be easier to share from the win machine using samba/smb, but I don't know how to set that up as ive never gone that way.

For an available-on-login setup in Linux, you most likely want to edit /etc/fstab on the client, with hostname below referring to the server...the entry should look something like this:

Hostname.domainname:/server/mount/point /client/mount/point nfs defaults 0 0

Hopefully some of this helps. It's late.

EDIT: it was late. I forgot the client-side mount point in the fstab entry!