I just use SSH+Rsync for everything. I traded two-way sync for minimalism and reliability. I’ve had nothing but headaches with anything else, especially Syncthing.

My Computer and both Raspberry Pi servers both run Linux and I have Termux installed on my Android phone so OpenSSL and Rsync are easily available.

I made a script that runs Rsync commands from files containing all the information which easily swaps source/target files so I can easily transfer in both directions with a simple command line option. It’s reliable and simple and I’ve had a lot less headaches troubleshooting the rarely occurring issues.