That seems normal to me, 30 days can be a bit short for a holiday, but over 60 days or even 90 days and then again within a month has nothing to do with tourism, so it is a correct decision of the immigration officer.
Just get the proper visa for your purpose of stay and you don't have to wait.
Visa exempt is for tourists, and I think with 3 visits of a month and one visit of 3 months in about a year time it might be difficult to make an immigration officer believe you are a tourist, but you just might have some luck.
It is up to you what cash you need to trade with. It is up to MFA to decide what they want from applicants for DTV, and I can understand they want to keep it simple. Just a direct access savings account is simple. When they accept different, what would they have to choose as limits? 2 days like your account? What can happen with that money in 2 days? Some applicants might just speculate with high risks. They just want have applicants have a stable financial position, that is also why some embassies want to see the money during a longer period and not just transferred to the account for the visa application.
But the fact you never need over $500 to trade with speaks for itself I think, obviously you see 500K as big money.