PX1172RH-HAT looks like a good choice for RTK and YAW in one package for ~$200. Cheaper and more compact than two ArduSimple boards, and a bonus - you only need to use the one serial port on the flight controller.
As long as it sends the right NMEA message for YAW to make ArduPilot happy.
I am reminded, however, that with two F9Ps we can use one for RTK position, and one for Yaw.
With this unit, we won’t know if we have a RTK Fixed Yaw only, or a Fixed Yaw + Fixed position from the base.
You won’t know when there’s an interruption in the feed from the fixed base.