You do not have to pay Thai taxes unless you bring money in from Thailand. So if you don’t work for a company in Thailand, get money from condo rentals, or make any money from any business in the country, you do not become a tax resident or pay any taxes there, no matter how long you stay
If you are from the US, there’s actually a treaty that prevents you from being double taxed
That's ridiculous. With the Bangkok bank craziness, lately some banks and branches have been tightening up. My advice, would be to keep trying banks in other locations, even if it's the same bank company you have already tried. Just use a different branch and you may even need to go to another town or city in the province.
If you meet all of the qualifications for the Non-O 90 day visa, you should be able to apply in country after you're married and registered., and then the extension after you've left 400,000 baht in a Thai bank account for 60 days.
To get married in Thailand, you can come in on the standard 60-day visa exemption like most other tourists.
If you run into problems applying for the 90-day in Thailand after you are married, you may need to see an agent to help the process.
Worst case, you may need to make a border run to a neighboring country, then apply for the 90-day Non O after you're married via the E-Visa website or an agent. Wait until approval (up to 15-20 days max), then cross back over to Thailand open a bank account, deposit 400,000 baht in the bank, wait 60 days, then apply for the 1-year Non-O marriage visa