have you been to Thailand on a previous visit and opened a Thai Bank Account in your name only?
If you have that, that's great, as it's become just about impossible to open A Thai Bank Account as a foreigner this year. If you're in Nakhon Sawan, you could try enquiring about opening an account (From my knowledge, there's very few foreigners living in that Province, so who knows if it will be easy or impossible). It would depend on if the Banks strictly follow their headquarters rules and policies. If this is the case, then from many comments I've read over the past, almost three years, it's not too hard to do the Visa application yourself without paying an Agent to do it for you. All you would need to do is go into the Immigration Office in Nakhon Sawan, which I believe from my search is part of the Royal Thailand Police Office precinct. Ask them for the list of requirements for applying for that visa, gathering the required documents and presenting them, along with the completed application form and fees.
If you don't already have A Thai Bank account, it's not quite as simple. In this case, you would be best to go to a neighbouring country of Thailand, such as Malaysia, Laos or Vietnam (wouldn't recommend Myanmar or Cambodia due to conflict issues) and apply for the Visa through the eVisa system of the Thai Embassy or Consulate there. Then, once you have the Visa, you can re-enter Thailand on that Visa and quite easily open a bank account with that. If you're already in Thailand in a tourist visa, you may not want to fly back to your home country (USA, it seems), so a short holiday in one of the countries I've mentioned, while waiting for the Visa to be approved, might be best.
Please note: I'm not currently living in Thailand, but the information I've given is based on knowledge built up over almost three years of reading posts by others in this and a couple of other Fb grouped, and a little bit of personal research also.
I am keen to know how you go and maybe keep in contact, because I have a Thai lady in Nakhon Sawan who I intend to marry as soon as I can get things sorted out at home (Australia). Feel welcome to contact me in Facebook Messenger if you want to.
what about "wasting your precious" money? As long as one meets the requirements for the visa, many will say it's not very hard to do for oneself. I find it hard to imagine that a foreigner looking to get a Non-O based on either retirement or marriage doesn't have the time to do it for themselves... after all, they wouldn't be allowed to work.
that's fine as an initial step which the OP may have already done, but if you have any experience yourself, you should know that the Immigration Offices in each Province have their own individual variations on what is required.
Question 2...with The extension based on 'marriage', once the extension has been stamped into the passport, any amount of the money can be taken out of the account. It then has to be a minimum of 400k in the account two months before applying for the next extension.
is very knowledgeable. I've only told you what I've read people such as those mentioned, say on other posts. I don't have enough knowledge to answer in more detail.
on the O/A Visa, each time you exit Thailand then come back in, you get stamped in for a further 12 months, so I'm that first year, you can exit/enter as often as you like. If you exit and re-enter just before the end of that first year, you'll be stamped in for a year, giving you almost two full years, depending on when your compulsory insurance expires as that determines the 'end' date.
If previously married and divorced, I'm not sure if just a divorce certificate is sufficient. Someone will correct me if I'm wrong, but I've read that a "Certificate of No Impediment" is required.
Thanks for the information in your first paragraph of this response. While it hasn't meant changing the way I had thought about my plan in what would be almost an identical scenario to the OP, I wasn't aware of that 2 month 'rule'. Is that 'rule' strictly adhered to across all provinces, or does it vary according to each individual Province, as some other requirements do? I did notice that you said " at any Immigration office".
apply for a Non-O based on marriage to a Thai in your own country or a neighbouring country of Thailand once you're married because there is a much higher chance of opening a Thai bank account once you have the visa, if not certain.