As I said, it will be a problem if they ask you for an onward ticket when you check-in for your flight to Thailand. It won’t be a problem if they do ask for one and you don’t have one. It’s easy to buy a refundable ticket, in case they ask, then cancel it after you clear check-in. Or, as already recommended, by an onward ticket online that is a real ticket that they automatically cancel after a short period of time. They cost around $15US. Good peace of mind.
All airlines serving Thailand are required by law to check for an onward ticket within the visa timeframe. Most airline clerks check; some don’t. Follow the advice already given.