@Dave *********
The departure field of the TDAC form is not mandatory, it can be left empty, and your return flight will not be a problem at the Immigration entry booth, in case they ask.
HOWEVER, it might be a problem with your airline. Many airlines will ask you for an onward travel proof out of Thailand within the first 60 days you will be stamped in. They will not allow you to board the flight unless you show either a fake onwardticket (which I do NOT recommend!) or a cheap budget airline one-way flight ticket for example from Hat Yai to Kuala Lumpur that can be booked for $40 Dollar. It does not necessarily have to be a return ticket.
You should contact your airline by email (do not call!) and ask them on their stance, then you will get some reply you can print out and take to the check-in just in case the check-in staff has a different opinion