After you submitted the application you can not do anything to change the application, until the Embassy eventually ask for additional documentation, which by the way, resets the visa processing clock back to zero, and then you should only provide the information the Embassy asks for, and nothing else.
No need to and it has never been the meaning that people should use agents to apply for DTV, hence the rule that you need to stay and apply for DTV only from outside of Thailand.
There is no need to support corruption by using agents, that is a clear goal of the government, in how they made the DTV guidelines.
And a agent can never so it all, since they can not travel to other countries and apply for you.
Take control of your life, and do proper research, then it is very simple to apply.
Start by thoroughly reading Embassy websites for the specific Embassy to apply at, and read the manual in top right corner of the visa application website.