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.
Yes. Some countries’ embassies still issue an affidavit of income which you can use instead of monthly deposits or banked money.
US, UK, Australia and Canada (probably others) will no longer issue these. So the only option if your embassy won’t issue one is to use 800k banked money for the initial visa and first year extension and after that can switch to 65k monthly transfers.
Tax issues are outside of the scope of this group, but afaik you only are subject to tax on income brought in to Thailand.
Many countries have dual tax treaties with Thailand and that means you’re not taxed twice, but you really need to get advice from a tax expert or accountant to work out any potential liabilities.