If I were to install a pressure booster pump, would it work / be on consistently or is it only triggered by specific water pressure ranges? A plumber told me it would only kick on if the pressure dropped below 30 or 40 psi, but the documentation I'm finding about different pumps doesn't indicate that that is the case.
Depends on who engineers the pump and controls. They can be either way. The most economical way would be with an expansion tank and a pump that runs only on demand. I have seen both systems, and installed both types.