If you are applying for your initial non-o in country, the 800k must be from a foreign source. If you got your non-o out of country or visa evisa, then it doesn't matter where is came from but must be seasoned for 2 months prior to applying for 12 month extension.
If applying in country, 800k has to be sourced from foreign funds and to be in the bank at time of application for non-o. If non-o was obtained out of country the 800k must be seasoned in bank for 2 months prior to application for extension.
I secured housing first. Then certificates of residency for bank account and mc&car drivers licenses (just got the 3 at one time so save trips later). Then to bank to open an account. Wire transfer the ฿800k from us bank to Thai bank. All easy. Can seem overwhelming but just go I step at a time. It took about a week to find a house to rent. But that might be harder and more expensive in high season.
dta's are treaties. You are a tax resident if the country you reside in for more than 180 days DTA's govern what is taxable income and any tax statements between 2 countries. USA taxes works wise income of its citizens, do the DTA with Thailand governs what each country can tax of the others citizens. Let's just age to disagree. That's how it's worked in each country I've been an expat in.