@Kevin *********
@Brandon ***********
has given correct information as he always does. When starting this process, for a lot of people it is quite daunting and maybe also confusing if you're getting information from multiple sources and I think this is what's happening with you. I don't blame you though, so if you haven't been doing research for very long and following posts on retirement visas in groups like this for long, I would recommend listening to Moderators and other group members who provide information that's consistent with that of Moderators. Though my plan is to go via the Non-O Visa based on marriage, the basic process is pretty much the same. I've been following these types of posts in 3 Fb groups for the past two years and see confusion fairly often.
'IF' you were to go to Thailand on a 'tourist' visa, then, to apply for the Non-O Visa "based on being over 50" (Retirement), within Thailand, you would almost certainly need to use an agent to help you open A Thai Bank Account (into which you would need to deposit 800k Baht which is needed to apply for the Visa).
The other option is to apply for the initial 90 day Non-O Visa (based on being over 50) from the eVisa website of the Royal Thai Embassy in your home country before travelling to Thailand. Doing it this way, in the application process you can provide evidence of having the USD equivalent of 800k Baht or more, in your bank account in your home country and at this point you don't need a Thai bank account.
If you get the Visa in your home country, as described in the above paragraph, then you can enter Thailand on this 90 day Non-O Visa.
Then, once in Thailand, open a Thai bank account yourself, which you would be able to do without using an agent. Changes over the past year or so, have meant that Thai banks, most branches at least, will not allow people on tourist visas to open accounts, unless an agent does it for them. They WILL however, allow a foreigner who holds a long term visa, such as a Non-O open an account on their own.
Once your Thai bank account is open, transfer your 800k Baht (or whatever the USD equivalent is), into your new account Many people use the "Wise" app to do their transfers to Thailand, because it's less fees and a better exchange rate than most banks.
Keep in mind that the 800k Baht has to be in your Thai bank account AT LEAST two full months before you can apply for your first 1 year Extension of Stay. Also, start transferring your monthly 65k Baht into your new account, so that you have 12 monthly deposits showing as International Transfers before you return to Immigration to apply for your 2nd Extension of Stay.
After your 800k has been in your Thai bank account for a full two months, and before your initial 90 day Non-O Visa expires, go to the Thai Immigration Office nearest to where you will be living in Thailand and apply for the 1 year "Extension of Stay" which will allow you to stay for another year.
Once your 1st Extension of Stay has been stamped into your passport, you can allow your account to go down as low as 400k during this 1st Extension. If you choose to do the 65k monthly income method, you must keep up the monthly transfers (showing it's an International Transfer) every month without fail.
Should you decide on using the 800k method (minimum), that money must be in your account 2 months before applying for an Extension and 3 months after the Extension is granted. For the rest of the year, you must keep a minimum of 400k in your account. Choosing the monthly income method, once you are granted the 2nd yearly Extension, all you need to do is make sure you keep up with the 65k monthly transfers shown as "International Transfers" in your account.