The iperf results show a pretty significant packet loss of 22%. Try lowering the bitrate until it’s consistently at, or close to, 0% and also test in both directions. The resulting bitrate will be what you can expect to realistically get rather than the raw burst speed.
10.0.52.117 is on a private subnet range, so is there a VPN between the two Syncthing endpoints? What type of link is it?