You might have misunderstood what the bank wanted. Think about it: How can they charge a 7,000 baht fee when other banks only require a small deposit when opening an account?
If they say "No" (mai-die) just go on over to the next bank. Repeat if necessary until you get what you want. The bank branches here interpret the rules as they understand them, though understandings differ.
Easiest for me is to just go to the desired telephone company office and buy it there. 7-11 stores have difficulty filling out the form that registers the SIM with the government (usually done with a Thai ID card) but must be filled it manually when copying information from the passport.
It is OR. After you get to Thailand, go to the nearest immigration office and ask what THEY consider proof of income in your case. (Seems to differ by office)