Take a trip somewhere for a few days before your 60 are up. That way you don’t need to extend and when enter again the new 60 day stamp will cover you for the remainder of your trip
Book your return flight for the 90 days and then go online and book the cheapest ticket out of Thailand within 60 days. You don’t need to use this ticket but it’s just there incase you are asked. Probably cost you £20 but worth it for peace of mind. You will then get a 60 day stamp when you arrive and then you can extend by a further 30 days