My last check-in with Singapore Airlines out of Brisbane I never even spoke to check-in staff as it's all automated kiosk now where they don't ask about visa or return flight.
So not a problem for OP. If issue just explain you'll get a 30 day extension. This method is pasted all over the Thai Embassy sites as a legitimate extra stay process. He also has a return flight so no cost risk for airline.
All these people stating you must have a ticket within 30 days are scaremongers who have likely never been rejected a flight, just repeating nonsense they heard on this forum 😉