PWM speed control of 24V DC Brushless Pump
You can't control the pump voltage , and you shouldn't. Not what they are designed for.
What you can control is the output resistance.  You need dc controlled adjustable valves installed on the output of your pumps. Brewpi, etc should have control for these built in and are relatively inexpensive on the typical sites(AliExpress).  Good luck.
Edit: There are tons of documented builds using this method on homebrewtalk. Not sure what platform or program you're looking to use for your panel but, unless you are coding it yourself, there's detail threads on the usual suspects like brewpi, craftbeerpi and raspberry pints.