Congratulations.

Go OS, get single entry (Cheapest) Non Imm O - Marriage visa for 3 month, make sure I fulfill all relevant criteria for extension of stay in advance (ie;

400k+, 2 months+), then anytime before the expiration of the 90 day single entry visa, apply at immigration of an extension of stay.

Apply within the last 30 days of your permission to stay.

You can get a 60 day extension on your current entry based on your marriage and at the end convert to a non-immigrant visa when you have seasoned the money long enough. But conversion often has to be done in Bangkok.

I could have then applied for the extension of stay without having to ever had acquired any visa?

No. You would have had to first apply for non immigrant visa entry stamp at Immigration in a separate action at a cost of 2,000 baht. That gets you the non immigrant entry that is required for one year extension of stay for marriage which can then take place later.