If you dont know the exact date you want to leave Thailand, just buy a bus ticket to Phnom Penh and just use that as onward journey
There is a misconception that IOs wish to see your return flight back to your home country when this is not the case. As long as you leave within the time permitted on the visa, all is well
On some occasions I was asked about an onward flight when I arrived on a 1 way into Thailand. I just say that I am considering extending my
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days at Jomtien but havent decided if I will yet, meaning I cannot book an exit until I know when Im leaving right?