@craig607 great to see the stepper functionality and managing it with the GCODEs though I wasn’t clear what the Marlin board is, and how it gets activated. Is the relay the power switch ON, or a pull to ground input.
This probably says how basic my view is, looking at powering, the 4.2V is nice as it maps to a LiPo battery, though LiPo are 4.2V decreasing to 3.8V. With 1.5As power draw the battery needs to be sized for the length of time the pumps can operate.
I’m imagining a use case of it being solar powered, so guessing at a separate LiIon battery pack, with probably separate charger and possibly solar panel – possibly all in the Marlin board? Just checking. Also, thinking typically need to measure the LiIon voltage to check its being charged.