USBs aren’t convenient enough. I’ve done wifi sharing via FTP, which didnt require WAN. Ftp was slow to set up. My bluetooth’s bandwidth seems very small?
A 1 click to install app across multiple devices, that only uses LAN, would be ideal. Right now, I just transfer files with element
As someone else said, Syncthing. It’s pretty much exactly what you’re looking for. We use it internally at my company where we use all major OSes and devices.