You can use funds in your USA bank for the application via evisa. Get your non O then come to Thailand and follow steps to open an account when you arrive. Deposit the required funds and then apply for your 1 year extension.
There’s always the danger of inadvertently falling below the 800k level if one or more transactions are not remembered. Many folks keep a second account for day to day transactions thereby safeguarding the minimum balance requirement
you’re in great shape if you already have a Thai bank account into which you can deposit the 800,000 baht before applying for your first annual extension of stay. Be sure to check how far in advance you’ll need to get your funds seasoned before applying for the extension