I'm using SFTP, so technically not a mount in th way that I'ver termed it.
File Manager in DSM allows for you to create the remote connection. I've chosen SFTP, but with multiple transfers, or large transfers, it can bomb out. The remote connection gets removed/hidden from File Manager and I would have to reboot or wait for a time out of that connection to start again.
If there was a client support, I'd be happier using that.
I've not looked into rsync, but I'd like to.
I've tried looking into Syncthing but can't fully work it out, tbh.