Can a Thailand DTV visa be extended in-country at 180 days?

I’m already in Thailand on a DTV visa and trying to figure out if I can extend my 180-day stay inside Thailand, or if I need to leave and come back.

A visa agency told me they can do the extension for me, but said if I go to immigration by myself, they will probably reject it unless “extra money” is paid to officials. That sounds shady, so I want to know what the real situation is.

Can DTV holders usually extend the 180 days by themselves at Thai Immigration, or do most people need to do a border bounce instead?

On the Thai Embassy site it says I can extend it 180 days + 180 days.

The site you linked to gives the answer (yes, ok to extend while in TH):

Extension of stay: 1-time for a period not exceeding 180 days per entry. After the maximum stay of 180 days + 180 days needs to depart the country and re-enter Thailand with the same DTV visa within the visa validity date