Anonymous participant the basic requirement is that you have the equivalent of bt500,000 in a bank account in your name. There are no exceptions to this no matter what route you take.
you will have problems trying to come back to Thailand as a tourist, as your record shows you are living here long term, and not really a tourist. You had a retirement visa extension, then a marriage visa extension, then visiting family, and now trying to get a tourist entry. It actually looks like you ran out of money, and can't afford to get the visa extension you should be on anymore. Expect to be told you can not enter Thailand until you again get the correct long term visa, as you are not a tourist. Thailand has lots of long term visas, as you know, but they all require that you have money to afford them. If you don't have the money for them then the simple fact is Thailand does not want you here, even if you are married. That is the reality you face when you try to return without using an agency.
The first thing she must do, and an agent can not help her with this, is get a right to marry notarized document from her embassy, basically that she is either single never been married, divorced with divorce decree documents, or widowed with the death certificate. After she gets this you can get married at a local amphur, with copies of her passport, copies of your Thai ID card, and copy of the house book you are listed in. After you are married to get the type O visa based on marriage she can only use the immigration officer in the district her TM30 was filed for with her living address. After you are go to the immigration office and get a list of documents and pictures needed for the visa, then the visa extension. Keep all the documents because after the first time everything after that is all the same except new bank letters and new pictures, unless she moves. It is quite easy to do. It just takes time, and there isn't really anything an agent can help you with. The financial requirement is different for a foreign woman marrying a Thai man, so be very aware of that when you check the requirements at immigration.
You need documentation of a complete treatment plan from your doctor, not just an initial appointment. Has your current doctor consulted with a doctor at Bumrungrad hospital yet?
the other thing they can ask for is your latest income tax filing. It is not a cumulative total in your bank. It is that the balance must be above bt500,000+ at all times.