I communicated with Yuri and we both agreed that the Pixhawk 4 will work just fine and Yuri said it even supports scripting. It’s just not the best flight controller to recommend to a new user. Your case is different and what you have will work. It’s not as capable as the newer models and if it becomes limiting later on it can be replaced. Waiting to tackle the remote operation by the new method is probably best for now until you learn more. Also. I think others will try this soon and we will all learn from that. If I wasn’t so busy this summer I would do it.
The starter kit you bought from ArduSimple likely has an SimpleRTK2B GPS board and a GPS antenna and you can use them, but you will need a second SimpleRTK2B GPS board with an antenna for the moving base. Please check on what you have in that kit.
Slowing down and taking this big project on piece by piece will be better.
Meanwhile I would order the remaining parts you need and do a benchtop setup with the Pixhawk 4, the radio transmitter/receiver you have, and the servos to to achieve servo movement on the benchtop. The control will be coming from the RC transmitter and going through the Pixhawk 4 set in manual mode using Mission Planner. That sounds simple but for a new person there is a lot to learn and a lot of things to marry together, including the Pixhawk 4 and your windows computer using Mission Planner. At first I would start out with the Pixhawk 4 connected to your computer via a USB cable and then later I would also integrate the SiK telemetry radio into your test setup and remove the USB connection. Even integrating these pieces together is a step by step process and you will learn a lot at each step. This is all spelled out in the Setup guide on the Ardupilot WiKi for rovers First Time Setup and Configuration — Rover documentation
Also since this will mainly be a mower building discussion we will move it back to the Best Gear for Starting Ardupilot thread so the other builders will benefit.
https://discuss.ardupilot.org/t/best-gear-for-starting-ardupilot-mower/