Put your availability and phone number at the very top. If someone has water coming through the ceiling, that's all they want to know.
Be upfront about call-out fees. Plumbers have a reputation for surprise invoices, and showing your fees, or at least a price range, is a quick way to earn trust.
Give hot water, drains and gas their own sections. They're very different searches, and separate sections help both customers and Google find the right thing.
Show you're local by listing your suburbs and linking your Google Business Profile. Local plumbing searches are mostly decided by the map results.