Anonymous participant this is likely the reason barring any other insufficient documents. Minimum 6 month course is recommended. You say "the site said", what site? Which gym and what is their DTV approval success rate?
Without details on your submission no one can give you any real help. So, please lay out everything you submitted, course details and provider, bank balance, account type details, statement length. Rejections are typically insufficient documents.
Typically US consulates are okay with 500k ending balance. Send the 2 month of statements you have along with a brief cover letter pointing out/highlight the pension deposits.
Just do it yourself, 10's of thousands of people do. Ensure there are zero mistakes in the passport data page upload after the OCR reads and propagates it. Everything else is pretty much answer questions and upload documents.
No it doesn't have to be your initially intended flight or date. I would definitely get a flight that corresponds to you starting your soft power course on time.
She's not currently/application time a remote worker, unless her current job has some remote aspect to it, that she can articulate, certify via employment contract, verify.
You're not telling us how you make income, are you completely retired, do you have a pension, how do you make money? Million baht isn't going to last you 5 years.