So the connection via SSH from Ubuntu to MacOS was possible. Why not establish a GUI-connection via Files. You can add other network shares at the + at the bottom of the shortcut list.
In the form add the address of the remote computer and use a sftp connection:
sftp://192.168.xxx.xxx (use the actual ip-adress where you see the xxx)
And that's all it takes.
Now you can work with the files on your Mac from Ubuntu.