Do I need an onward flight ticket when moving to Thailand on a DTV if I have a TDAC?

You don´t need onward ticket on a DTV

What!!!!!

Just tell immigration that your applying for a retirement visa . It has worked for me last February and in June my mate came with a single flight and we just told them he was probably going to get an extension of stay and that we were not sure when we were going to travel to Cambodia. We did it as pleasantly as we could and they smiled and stamped the passport

Read the TDAC form. It's not a permission. It's just confirming your travel plans.

The TDAC is not any of those things. It's an arrival card. It's not a permit. It's not a visa. It's how the Thai government is getting stats on arrivals. It has nothing to do with your visa. It has nothing to do with your stay in Thailand. It has to do with your arrival and that is it. Thailand Digital ARRIVAL Card. It is replacing the paper form you used to get on the airplane. Whether or not you need an onward flight has nothing to do with TDAC, and everything to do with your VISA.

With a DTV you should not need proof of onward travel based on reports of others.