I'm at this level myself, look for a constant current driver, in stepdown configuration, have the micro measure the input voltage, turn on the constant current for a second, see how badly it drops, if it remains high enough, then leave it on as needed, if it drops, try again in a minute,
Or if you want to avoid jogging the pump, switch on some load resistance (hey solar is free?) and see if it drops, then decide to turn on your pump,