There's no such thing as a 30 day extension on a non-B visa. Your only option is a 1-year extension based on work if you have a non-B visa. People on internships generally cannot get extensions and have to leave Thailand to get a new visa.
You're not going to be able to get a bank account if you're here as a tourist, so that's going to be your biggest problem. The first thing you need to start the process is a bank account, because you have to show 800,000 Thai baht in a Thai bank account in order to apply for the retirement visa.
You will likely have to use an agent in order to get your bank account, but due to various government crackdowns on illicit activity, it's become increasingly more difficult and expensive to get a bank account. In just the last few weeks the price of a bank account through an agent has gone from 5-6,000 baht to 12-15,000 baht.
Once you find an agent, you can decide to either just get the bank account from them, and then do the rest yourself with immigration, or you can decide to pay them more money to do the rest of the process in obtaining your visa as well.
One alternative you have is to go to a nearby country to Thailand and apply for the retirement visa through the Thai embassy there. Then you would have the non-O visa and could use that to help you open a bank account when you returned to Thailand. That would then allow you to do the entire process yourself.
No, you cannot change from one to the other. The bank is just looking for a reason to say no. If you had an evisa they would find another problem. Move on to a different bank branch
kind of. For the initial application both will accept foreign bank accounts. For extensions in Thailand the retirement visa money must be in Thailand but DTV extension officially says it will accept in Thailand or foreign. There are reports of some immigration offices only accept a Thai bank account for DTV extensions though.
Yes visa on arrival is possible. It can be a long wait, and it takes an entire page in your passport. If either of those is a concern, then you could look at getting an e-visa which only takes
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of a page. Evisa is only accepted at certain borders though.
the money is not for living in Thailand. The money is for you to show you can take care of yourself in the event of an emergency so you won't become the Thai government's problem.
It's the DTV. It actually costs 10,000 baht if you do it yourself. And while it's valid for 5 years, you can't just stay in Thailand like you would with a retirement visa. You'd pretty much need to leave every 180 days and then come back for a new 180 day stamp. There's also no guarantee that in the future they won't start asking you for proof that you're still doing what you initially said you were doing to qualify for the visa. Many embassies have started tightening up the requirements and immigration might start too at some point.
The retirement visa/extensions will give you a much more secure ability to stay in Thailand.