certainly one of the main criteria is that you are a financially stable (thailand wants people spending money) and if you just barely make it with your 500k its not going to make a strong application. The embassy may very well asked to see your last three/six months statement just to see what kind of money is running through your account.
Have you actually gone to Rayong office, explained and asked? And in theory your plan will work, except they may ask for your lease agreement, bank letters are good for a week, they will likely want to see the year statement/letter, your extension will be approved and inked in right then and there.
no such rule, it's a myth. But, if immigration thinks you are milking visa exempt stamps to live here, then prepare to be pulled aside. In the OP case they are a genuine tourist staying a few months at a time, no issues.
you can definitely apply for an extension in country, but it's a dog and pony show. Good luck. Don't leave it to the last minute in case things go south and you need to bounce.