When can I apply for a Non-O visa in Thailand with a marriage stamp after having left the country?

So, you were in Thailand on Non-O based on Marriage for 90 days, but you already left Thailand without an extension or re-entry permit, right?

You are mixing some. You need to have your 400K in your Thai bank account to be able to apply for an initial in-country Non-O based on marriage to a Thai national. There is no monthly transfer method to meet the financial requirement for an in-country Non-O visa. You can apply for it anytime after entering Thailand as long as your stamp remains more than 15 days (calendar or working days depending on the province).

I doubt you can use monthly international transfers for the very first yearly extension either. But this is what you need to check with your local immigration office.

Q. When I return to Thailand when is the soonest I can apply for the Non-O? September 6th? Or is it October 6th?

-> The day you enter Thailand is the soonest day to apply for the initial Non-O visa at the local immigration office if you still have 400K in your bank account. Because you cannot use monthly transfers for this.

Sounds like bad or good, it's the fact, unfortunately.