18+ annika, I spent some weekend time replacing Resilio Sync with Syncthing. It was a little finicky getting it working in a Docker container on my Synology NAS but now it's running across 2 Linux hosts, NAS, Windows, macOS, and Android.
I'm a long time paid user of Sync (I have a BitTorrent Sync email from 2013). It's not had any updates for almost 2 years, and it feels abandoned now.
Syncthing seems solid. I'll miss partial sync.