I used Syncthing for many years, absolutely rock solid. I wanted to use it to replace Dropbox but renting a VPS with enough storage was surprisingly expensive - you’re no longer in the loss-leader section of their offerings.
Couldn’t you mount a Hetzner Storage Box via SAMBA (SMB) or FTP to your device and then point Syncthing at that network storage? The box is €3.20 per month for 1TB of storage.
It’s a network attached storage with some FTP/SAMBA etc managed by you via a web interface.
Think VPS with lots of storage, but with file-storage software pre-installed and configured and only ability to change users. I don’t know how else to explain.