It all depends on your finances or if you want to save baht (and spend a LOT of time) and jump thru all the hoops. Agents assist with everything in Thailand - they have the connections to simplify/expedite everything. I'm told they do it legally/in an allowed manner otherwise they wouldn't be in business.
Simple example? Immigration offices - some foreigners stand in line for an hour or 2 to get in the door and then they're lucky if they do something simple in one day. Others pay an agency and are walked in front of everyone else.
For retirement O - if you use an agent, they can satisfy this requirement as part of their service (for an additional fee of course but rather than locking up that much money, it may be worth it).
It's a matter of paying to go the simple route and having everything done for you. Besides all the work for the visa, they can also open the bank account and bypass the 800k requirement as part of the service.