You might find it easier (if you have the means) to increase your 400k to 800k and apply for the non-o and extension based on retirement. Much less paperwork and you're not tied to your marriage for your extensions (or if you're wife should pass). Less paperwork and easier process.
Depending on how long you plan to stay in Thailand, you probably don't need a visa or any extensions. Each visa exempt entry currently gives you 60 days which can be extended by 30 more if you need it.
800k balance must stay for 3 months following extension and 2 months prior to next. At no time can it fall below 400k. If you don't have 800k in savings, it would not be wise to relocate to any foreign country...
Their reason is that they don't see a compelling reason for her to return. There are 100's of thousands of -overstays"living illegally in USA. They're trying to shut it down. Your easiest for would be a fiancee visa (don't remember the number). But then your have to marry her within 90 days and commit to sponsoring her.
You might try doing other international travel with her to Europe, Australia, Japan, Korea, etc. and showing that she returns. That might help. But with current administration, might wait till 2029 before you apply again.
technically it is illegal. Maybe not for the agent, but for the immigrant who doesn't meet the qualifications for seasoning and minimum balance requirements thereby technically invalidating the extension. Unlikely, but not zero risk.
until that agent is made an example of and every visa with their initials comes under scrutiny. Very unlikely but still possible. Why not just follow the rules and do it in your own?