I recently started using SyncThing, it seems just perfect to share between two people. Maybe I'll put it on a server as well so we can sync without being online.
It's definitely pretty nice, but the ergonomics of it for someone that's not that good with computers can be a little hard. I've gotten synced folders into bad states before that took a long time to fix. It's also kinda awkward having to send over a nominally private and very long ID string to set up the share in the first place.
I stick with Resilio for this reason. For over a decade now it had been a 100% reliable fire and forget tool.