OX was only really doable from the Sydney consulate as it was the only one in the world where you didn’t have to prove 3m in a Thai bank account. No idea if it’s still the same since they’ve changed to evisa.
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.