How to run multiple `socat` and forwarding both TCP and UDP traffic?

I want to forward Microsoft Remote Desktop on a Linux Server, assume that the remote Windows host is 192.168.1.100, I want to run:

socat TCP4-LISTEN:3389,fork TCP4:192.168.1.100:3389
socat UDP4-LISTEN:3389,fork UDP4:192.168.1.100:3389

It works fine when I start the service by hand in the shell, but I want to start it with systemd, and log into /var/log/socat-rdp.log. After some trials I can run the service like this:

[Unit]
Description=Socat RDP Forwarding Service
After=network.target

[Service]
Type=forking
User=root
ExecStart=/bin/sh -c "/usr/bin/socat TCP4-LISTEN:3389,fork TCP4:192.168.3.153:3389 > /var/log/socat-rdp.log 2>&1 & /usr/bin/socat UDP4-LISTEN:3389,fork UDP4:192.168.3.153:3389 > /var/log/socat-rdp.log 2>&1 &"
ExecStop=/bin/kill $MAINPID

[Install]
WantedBy=multi-user.target

This configuration starts and runs normally, but when I stop the service with systemctl stop socat-rdp it returns error code 1, although socat process is killed as normal. Are there any better solutions?