The goal of that banked money is to be a reserve for expenses (medical, emergency travel, family emergency). If the banks block the funds, what's the point of having it at all. Get hit by a bus and can't pay the medical expenses because the bank has a hold on your funds?
Passport, tm-30, passport photo. Residency certificate are type specific so your need separate ones for banking, drivers license(s), vehicle purchase/transfer/registration. RC's are valid for 30 days. I take along my lease agreement too.
Depends on your country and their DTA with Thailand. Depends on the source of the money. Depends on if you've been in Thailand for more than 180 days in a calendar year. So the answer is "it depends". Giving more complete info will get a more definitive answer.
you don't understand the difference between tax residency and tax liability. You never lose your liability for taxes in USA even if you are a tax resident in one or more other countries. You are wrong.
In theory, if you moved here and deposited before July 1, you would be a tax resident in the current year and could be liable for tax. But it depends on the source of the 800k. If you can show it came from social security then it wouldn't be taxed according to the DTA with USA. Easiest is not too move till after July 1, then you won't be a tax resident for 2025. Given your writing this on jun-30, assume you won't be a tax resident.
If you're coming in a non-o, you'll basically need to leave the 800k in a Thai bank for 14 months (technically for 3 months following your first extension with balance not taking below 400k, then back to 800k 2 months before your 2nd extension. If you have 12 consecutive deposited of 65k prior to your 2nd extension you can convert to the 65k/month method and can take it the 800k.