There is no requirement to have a bank account in the province you’re applying for a visa/extension. There probably would be a need for a TM30 for that province but as we know nothing about what they are trying to do it’s hard to advise.
A METV is valid for 6 months from issue. On each entry you’d get 60 days and can extend once for 30 days more at a local immigration office.
In theory one would get you to September but seeing as you said you want to maximise your time from September you might want to enter exempt (if applicable) or with a single entry tourist visa if not and get the METV for September.
You’re going to need to provide way more info to get anyone to be able to answer you.
What is your nationality, what is your age, Isan may suggest you’re married to a Thai (although that’s just a guess - I can’t see why anyone would who isn’t would move there - subjective for sure).
No one can answer your question without way more information.