Can I extend my 90-day non-immigrant visa after depositing 800k in a bank?

You can enter Thailand on your 90-day non-immigrant visa starting from 2 July, and once you arrive, your 90-day count will begin. As long as you open a Thai bank account and deposit the required 800,000 THB within the first couple of weeks, you should be able to meet the requirements for a visa extension. Just ensure that your money is in the account for two months before applying for the extension.