I also see there is options for 8 different predefined motor speeds. I would see if that will fit your situation. Water pumps do not vary output/pressure linearly with motor speed.
If the high spots are not getting enough water and you increase the pressure (flow varies with the square root of pressure, 20% more flow requires 44% more pressure), will that not flood the low spots? Sounds to me like you need a system of throttle valves to equalize flow to all spots.
Can you have a flow sensor rather than a pressure sensor? The flow is what you want to makeconstant, using feedback from the flow rate is perhaps more stable than trying to attemptfeedforward from the pressure.Presumably the irrigator head moves at a constant rate so this doesn't need to be compensated?
Something like this might work with the proper resistor and capacitor values fed to the VFD analog input.http://sim.okawa-denshi.jp/en/PWMtool.php