What I did, and have been doing for a long time is I regularily rsync all my devices to my server. (Actually I do it nightly via cron but of course you can do it manually too whenever you desire...) Then, when I need to copy stuff from one device to another, I just do it on the server and rsync the changed /home/nemo directory back to the device it belongs to.

So, basically I have 3 different users for backup on my server, "jolla", "nexus" and "xperia". For each I have set up login via ssh keys, so I just need to do for example "rsync -av /home/nemo/ jolla@myserver.mydomain:/home/jolla/" on my Jolla sbj1 to backup it.
Then, to copy for example the contacts from Jolla to Xperia, I just copy content of /home/jolla/.local/share/system/Contacts/qtcontacts-sqlite/* and /home/jolla/.local/share/system/privileged/Contacts/qtcontacts-sqlite/* to the appropriate places under /home/xperia/ and do "rsync -av xperia@myserver.mydomain:/home/xperia/ /home/nemo/" on the Xperia...

One thing to remember; by default your nemo account does not have read/write access to the directories owned by the "priviliged" account. To go around this without needing to devel-su yourself you need to add the nemo user to the priviliged group.

This might be seen as a potential security weakening but the effects are negilible as long as you have strong password for ssh and always use automatic device locking to avoid people messing up with your device...