You will need a onward ticket within the 30 day visa excempt, since you have only got permission to stay 30 days, and have to show that you will comply with that.
There is a much better and cheaper way to do it.
Go to
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and apply for a 60 day, touristvisa, you get in 7-10 days at the most.
Then you don’t have to use holiday time to travel to a immigration office, waitingtime, and uncertainty if they will grant you the extension.
Doing it the way you described, may give the immigrationofficer the impression that you are a dishonest person, because visa exemption will grant you 30 days in Thailand, but you show signs of not intenting to leave within the 30 days, he can grant you.