For the purpose of getting this prototype made, should I just shoot for a pre-made buck converter?
Well, that's an option. Then you need to switch the output somehow, perhaps using a mosfet arranged as in the second circuit above. Then you're only two components away from a fully speed-controllable switching drive. But if you can live with the pump being on-off only, then switching a ready-made buck-converter will work with simpler software.