From Thai visa centre: Your visa is valid for 6 months. You can enter Thailand as many times as you wish before the validity date expires, and you'll be stamped in for 60 days each time. It goes without saying that you must meet any entry requirements in place for each entry. However, if you would like to stay in the country constantly longer than 60 days, it's simply extendable at the local immigration and you pay the fee of 1,900 THB.�.
You will be ok. However, l am flying to and from Vietnam for 5 days. With a METV.
l fly with Emirates all the time they don't require a return ticket for check in never been asked and l double checked with them before and neither does your TDAC.
Just say holiday. Don't need a return flight not even for TDAC. You can sort it when you need to in Thailand. If you want feel at ease get a multi entry fir 6 months. Or get a cheap ticket for about 20 dollars to leave (p. Me to get tge link). Better still go immigration not a problem. They won't even bother pal.