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.
not all immigration offices will let you get an extension shortly after you arrive. Some will tell you to come back on the last week. Check with Immigration first.