Check out SyncThing for a peer2peer (device to device) solution which doesn’t necessarily need a server, but having an always-on device like a server is still great for using Syncthing as well. It’s easy to use, only slightly more involved than setting up Nextcloud or Dropbox or whatever. But all done via a web-based GUI. It works surprisingly well, stable and conflict-free for the complex syncing it has to do all the time. Basically you install SyncThing on all devices you want to keep in sync, and they will find each other via their IDs when they are online, and automatically sync all their directories which should be synced. Of course it’s open source and cross-platform too.