Yes you can. You could have over 10m in the account and spend at will. Immigration only want to see that you have at least 800k at the time of application.
You are referring to a special visa waiver scheme that a few countries have with Thailand. That’s not a tourist visa. It’s a visa waiver. As a Korean if you bought a tourist visa prior to arrival you would get 60 days. Not that anyone would want to waste their money doing so.
Had a person today at immigration with a 20+ day overstay because they bought a “90 day tourist visa” prior to coming and wanted an extension. They hadn’t realised that a tourist visa is valid for 90 days from issue but gives you 60 days on entry.
There is no 90 day tourist visa. You’re either on a 60 day tourist visa or a 60 day visa exempt stamp. Either option allows a 30 day extension (except for a few countries).