@Jackson *********
That is a tougher one to give a definite answer to. In the past (pre-covid) hardly anybody ever bothered about the existence or the date of the return flight, and if they did, having one within the timeframe including the one 30 extension was fine. BUT: During COVID times there have been mixed reports. Sometimes people were denied boarding by airlines, sometimes they were denied their COE if they did not have a return (or more precisely) ongoing flight within the initial timeframe they are entitled to enter. So, I would not risk it and initially book a return or ongoing flight leaving within the first 30 days. Almost all airlines allow for flexible rebooking these days, so you could change / postpone that flight once in Thailand. Or buy a throw-away cheap or a fully refundable flight, accordingly.