[otachack]

I use Resilio Sync which used to be proprietary and it's been seamless. I do sometimes have to make sure the active device I'm on is synced before accessing the database but most of the time it's fine. I use my phone, laptop, and desktop as devices. With Resilio, there's a nifty option to have an encrypted store for devices you don't trust to read but trust enough to store (think like your parents computer or work computer)

SyncThing may also work for a open source solution, but doesn't have that nifty latter option I stated above, at least not out of the box.