Mavproxy forwarding telemetry to CrossBow Mini via Raspberry pi

I’m trying to setup a raspberrypi 4B to forward mavlink messages to my CrossBow Mini antenna tracker. I think my problem is the serial port on the pi, but I’ve done all the recommended things.
enable serial, disable login, disable bluetooth using btoveralay in /boot/config.txt, blacklist bluetooth
But when I run this: --master udp:pi:14550 --out /dev/serial0:57600

The crossbow doesn’t receive anything. Mavproxy is working because it responds with this:

Connect udp:pi:14550 source_system=255
Log Directory: 
Telemetry log: mav.tlog
Waiting for heartbeat from pi:14550
MAV> Detected vehicle 1:1 on link 0
online system 1
AP: ArduPlane V4.3.0dev (e2a132ed)
AP: Tims-MacBook-Pro.local
Received 1576 parameters (ftp)
Saved 1576 parameters to mav.parm
Flight battery 100 percent
AP: PreArm: Fence requires position

I know the tracker can receive telemetry because I have got it to work by connecting a SiK telemetry receiver to the same pin and got telemetry no problem.
Any suggestions?