Going through an agency is not 100% ok because they tend to go around the law.

What I would do, is to go directly at the Immigration office and explain your situation to them.

If you are polite with them, they usually try to help, They can easily extend your visa for one month (1900 baht) in order to reach 60 days you need with 800k baht on the bank account.

I don't know exactly you expiring date, but maybe 30 more days can do it.

I used xe.com since many years. Best exchange rate in real time and very low bank fees.