In my last playthrough, I used the strategy of using enough buildings to completely consume all available input. So, if I had 240 iron ore, I would make enough smelters to completely consume that iron ore, enough plate constructors to consume the ingots, and enough rod constructors to consume the ingots too. While it’s impossible to keep both the plate and rods running fully at the same time, I can guarantee that my limiting factor will be ore input regardless of the ratio of plates to rods I am pulling from the system. That property can be chained to more advanced parts, so when I make something like modular frames, I can still guarantee I’m input limited, not building limited, by just using the existing plate and rods systems.
I got it working by using a difference in head lift. Build a series of liquid storages, one on top of the other; hook them up in series. Connect fresh water to the bottom of the tower. Make sure that the fresh water line doesn’t have enough head lift to fill the highest container (you can move your pumps back or make the tower taller to do this). Connect waste water to the tower, with plenty of head lift. When the water level is below the limit of the fresh pump, both fresh water and waste water can enter the tower, and it gradually increases. When the water level is above the limit, fresh water stalls but waste water can still enter, and the water level decreases. As a result, the water level hovers around the limit, and your machines can keep running.
A simpler method with just limiting fresh input with a valve will stop working if the machines aren’t producing constantly, since the pumps will keep bringing in fresh water and fill the pipe when the machines are idle.