GPS RasPi RIO-AHRS

Looking for a solution to the following problem. The application is for an autonomous vehicle (typical four wheeled chassis, driven by two DC motors attached to two wheels) to drive to the starting point and then execute a CCW contracting square (possibly put a radius on the corners?) pattern. The data entered up front would be:
1. Coordinates of the four corners in Lat & Long
2. Track spacing (distance between adjacent parallel paths)
3. Vehicle speed (could be in MPH, FPS or any unit)
4. Coordinates of the starting point

Or I could do this project with a stepper motor attached to the steering of an existing self-propelled vehicle eliminating the need to use the two DC motors for propulsion and “tank” steering. This may be easier?

I’m assuming that the software would using the heading data from the GPS to determine if it was headed in the right direction? Making corrections for deviation off track line would be excessively complex at this point as a series of waypoints between the two end waypoints would need to be calculated and the delta calculated. Although it would need to hit the corner end point to “reset” itself otherwise the errors would be compounded and the accuracy would go down the drain leaving gaps in the track spacing for subsequent paths.

I have a USB GPS unit that will feed NEMA sentences to RasPi. Also have a RasPi and RIO-AHRS board to start with.

Any and all help would be appreciated. Or is anyone willing/able to write the code for compensation?

Thank you.