General Steering Module control

Newbie, so the question will be simple for some, but for me, I am just missing something.
I have created a 4 wheel drive mower build from 4 wheel chair motors connected to a motor controller and everything works great and provide the solution I wanted to solve of mowing grass in a very muddy location to help it dry. It would not support any type of riding mower and I did not want to stomp through the mud to mow.
Now I am working on a Dixie Chopper with hydraulic motors that has steering linkage very similar to a mower with transaxles. I have removed the linkages and feedback springs with the connections working as expected.
I have servos on each of the direction arms for each side wheel and a servo on the throttle (motor RPM adjustment), a relay on the starter and kill switch.

However, the issue I am having is forward motion. I want to control forward motion much like a joystick. That is right hand stick up and to the left motor to move faster than the right motor. full to the right causes the left motor forward motion and the right motor backward motion.

Sounds simple, but the issue is programing the the PixHawk 2.4.8 with a Jumper T18 Pro radio to listen to what I am telling it to do.
Left motor is channel 1 and right motor is channel 3 with function 73 and 74 respectfully.

Any help would be appreciated and with gas at over $5 per gallon, I’m not ready to burn it to the ground.