* fast as possible
* LAN, but also WAN when necessary
* in fact how about multi-protocol, and use the fastest protocol that's available on both devices, it's 2025 people!
* work for sending a gargantuan amount of files with possible interruptions (essentially, be rsync)
* be battery efficient, it's 2025 people!
* don't rely on a centralized cloud server, just connect the damned devices to each other
If you think you can hack this together, you should write it for Gnu Hurd because they'll release that well before you're finished