I did mine in Thailand 2 years ago and also did not pay anything to agents. I also extended it and sailed through in 30 minutes, no corruption, no agents, I just provided the evidence they requested.
The most difficult part is opening a Thai bank account to deposit the 800k.
Seems like people need to pay an agent to do this now, I managed without an agent 2 years ago.
After that it is a case of going to the immigration and asking for a list of requirements and providing the documents required. It seems complicated, but it is not difficult.