if he plans on doing it all himself then it’s unlikely he’d manage it - though it depends on the Immigration Office. Last person I know that converted from TR needed 8 visits to the office as they kept asking for more documentation.
You’ll need to get a Tourist Visa (TR) before entering then, yes, that can be converted to either a Non O for Retirement or a Non O based on Marriage within Thailand.
using an agent is not illegal. The visa obtained is a valid visa. It’s a typical Thai ‘grey area’ and the way Thailand operates at pretty much every level. Plenty of people who fully meet the requirements of Extensions of Stay pay an agent to avoid the paperwork and queuing at Immigration. They are doing nothing illegal and nothing wrong. It is perfectly valid. As for ‘repercussions’, I have not heard or known of any problems with authorities for the many thousands of people using agents for ‘Retirement’ visas in 35 years of visiting Thailand.
The hysteria by some on here where agents are concerned is hilarious.
I’m not suggesting everyone should use an agent but it’s a perfectly valid option for some.
I’d like to correct some false information in the replies. (One person that’s replied gives wrong information a lot!).
First, you can now get a Non O visa ‘for Retirement’ in Australia. It’s only been allowed recently but it is now on the RTE Canberra website.
Second, you DO need Health Insurance (400k baht inpatient /40k baht outpatient) for the initial visa application in Australian. It’s only for annual Extensions of Stay that insurance is not required for the Non O.
As for your initial question, the Non O-A is the only visa that will give you 7 months or more in Thailand without needing extensions etc. It does require paperwork and 3m baht insurance.
The STV is ending and isn’t really suitable.
The Non O is for 90 days and then you need an Extension.
The only way of doing this without money in Thai bank is to use an agent. IF you are prepared to do this then you can also just enter on a TR and pay an agent - though it’s not cheap.
Officially, yes you do need to submit a new TM30 each time you leave the country and return. Some immigration offices might tell you ‘no need’ when you turn up to do it but it’s not consistent, so best to go and do it.
The visa is for one entry (unless you specifically got a multi-entry in the first place). If you leave Thailand you will need a new visa to re-enter (unless you qualify for Visa Exempt entry of 30 days - you will if you are from USA).