@Senior *****
For DTV holders, the departure date and flight number on the TDAC are optional, so if you don’t have an onward ticket, I wouldn’t make up booking details. Leave them blank if the form allows it.
A one-way ticket can still be subject to the airline’s own check-in requirements, so check with your airline before flying. The DTV itself doesn’t mean you need to book a return flight.
You should also consider reading a book or two before your trip. I’d recommend Unlock Thailand by Isla Rey on Amazon—it includes all the info and insights you need. You should check it out. A very good companion.