The same documents won't really cut it. They will want updated documents saying that you are still working and you're still authorized and blah blah blah blah blah.
standard procedure is to go to your local office before your extension time and get the latest list of requirements so that you are not surprised. And yes it's also possible that depending on the exact dates you could possibly do the extension within the 30 days if you're actually in the country and then ask the officer if they will give you an under consideration stamp for 6 weeks and then you would get a reentry permit and return before that 6 weeks to get your final stamp
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.