My experience is that "fit for purpose" engineering is almost always the best solution.
I have the same situation at my farm. My conclusion was that the DC solar pumps were the best option. They are not cheap, but they are optimized for your application.
I don't know why your first solar pump could only make 3' of head. Most of the good manufacturers have a wide range of pumps with well-documented pump curves so you can get the right one for your conditions.
OTOH, you also learn something after putting your designed system into actual operation. If your DC treadmill motor is running your centrifugal just fine, then run that to failure. Document your performance to see if that is enough rate for a summer drought period. If the design needs any tweaks or improvements, then you can do that on the 2.0 version.
Good luck on your project.
