Syncthing is available to download to any pc, phone or via docker container.. It doesn't come witj OMV but you can add it via docker command line or Portainer GUI. you assign which folder you want sync'd , it gives it a unique hash ID, on other devices you enter the ID, or search for synthing host. It then connects and will prompt on host to allow/ disaaloe sync to certain devices. Any new files on either end are replicated. You can see rules on if one is read only, and what you want to do with version changes. i.e. on my PI it acts a master (which is not the correct term) and if changes via other devices populating to the PI then it folderizes old versions of files so that a mistake change that was saved on my phone and replicated to the PI doesn't remove the original totally. (Good backups periodically could also solve this)