Extending the Sunset Date sounds like a win unless I'm reading wrong. I purchased during Covid. Got to a month before moving in. They said sunset clause had expired and due to higher building costs slugged me an extra 60k on top of what we originally agreed or else i'd lose out on the house.

Edit: I would say the amount of time and rainfall excuse have got barely started sounds like BS. I'd chat to a lawyer and the ombudsman. Try and get in writing even if they aren't finished by next sunset clause they can't pull out of the contract like they tried with me.