Just get the Non Imm O before you arrive. You shouldn't have problems opening a bank account. You wont need onward travel to board your flight or enter Thailand, and you won't have the extra trip to an Immigration Office to apply for the Non Imm O in Country
It makes NO difference to the visa and the fact that you DON'T need to stay out of the Country for 5 days. You'll be here 360 days and tax resident anyway.