Anonymous participant remember that Vietnam embassy, and many others, want the money in your personal bank account for more than 3 months, and is why most get rejected there.
Just because you become a tax resident does not mean you owe, or have to file taxes on a DTV. You are on a tourist visa, and tourists don't file taxes no matter what anyone tells you.
Transfer that amount in by doing a SWIFT transfer. This gives you an easy to get FET, foreign exchange transaction, form that you'll need if you ever want to take that much back out of the country easily, much simpler than the multiple of Wise transfers needed.
For the initial type O 90 day visa there is no time limit on how long the equivalent of bt400,000 has to be in your personal account. The two months time period only applies when you apply for your one year extension inside Thailand with your Thai bank account. At that point the bt400,000+ must be in your Thai bank account for at least two months before you apply for your one year extension.
there is no tax on the bt800,000 you bring into the country. Thailand assumes you paid the appropriate income tax on it before it was brought into Thailand, until you prove you didn't. It is also best just to have your SS payments direct deposited into your Thai bank account. Thailand isn't going to steal, or tax it. If you are going to live here then live here, cutting as much ties as you can with back home. Any money you bring into Thailand, Thailand assumes you paid taxes on it, so isn't concerned about any money you bring in, as Thailand and the US have the best dual tax treaty.
on the Thai e-visa website in your home country apply for the type O 90 day visa based on retirement. Because you are both over 50 you will both need the equivalent of bt800,000 in each of your bank accounts, and yes thinking long term you will both need separate accounts. Not all embassies accept joint accounts. Once in Thailand for convenience you will both want separate bank accounts for your everyday shopping needs, as you can not have two debit cards on the same account. You each need a long term visa to open an account here. Once you are here on your type O 90 day visas you both open Thai bank accounts, and transfer by SWIFT transfer the bt800,000+ into each account, as you need these Thai accounts and money to get your one year extensions as soon as the bt800,000 has been in the Thai accounts for at least two months. It is all pretty straightforward, and the easiest visa, and yearly extension to get. This is how it is done now, because of changes preventing anyone on a tourist entry from getting a Thai bank account. The type O 90 day visa you will get is good for 90 days to enter Thailand, and stamped in for 90 days when you enter. As it gets closer to when you want to come here I can give you more up to date at that time details.