Thousands of people come into Thailand every day for business meetings, factory visits and IT projects for example to give training. Lots of western people but loads of Japanese and other Asians . Many do it visa exempt or with a business visa. But I would say what some small business billing Thai companies directly would require you to have a work permit. Definitely not allowed on a DTV regardless of where the billing is done from.
DTV must be issued outside of Thailand. For the Non O based on retirement this can be done at most local immigration offices. You don’t need an agent. Put the ฿800K in your Thai bank account and get the handout of the other requirements from the local immigration office.