If you put that on your visa application, most embassies will either reject it, or not process the application until you're 3 months away from traveling.
Visas are meant for traveling to a country. The way it's meant to be done is by getting the visa you want from a Thai embassy before traveling to Thailand. The system is designed around doing it this way and it's the simplest method.
It's only the person offering the AirBNB that might be breaking the law. Not the customer.
But if you think for any reason you might need to go to immigration during your stay, message the person offering the room before you book to ask if they'll do TM30 for you. I always do that.
If you arrived with a non-O visa at the beginning of June, that would mean your initial 90 days would go until the beginning of September. You would apply for a 1-year extension which would go until the same date every year you apply, which would be some point at the beginning of September.
When you apply for an extension based on marriage you get the under consideration stamp, which can go past the end of your existing extension. The under consideration is then your temporary permission to stay and is an advance on your future 1-year extension which you'll receive in full after the consideration period.
Based on the times you've indicated for your initial entry and extension, it sounds normal to me. Your annual extension will always be the same date every year.
most people for DTV seem to go to Vietnam, though they seem to be getting more strict there too. I've heard people start recommending Indonesia more because you can stay in Bali while waiting for your application. A few mentions of Philippines too.
Malaysia is very much not recommended as a place to apply for DTV. They will ask you for documents you've already sent them at least 3 times. They will be VERY strict on the requirements. And they'll take approximately a full month to approve or deny.