What are the requirements and considerations for applying for and extending a STV visa in Thailand, including accommodation and insurance details?

AirBnB - are not legal in Thailand. If you are going to use them, say the host is a friend. So far, I have not any hosts that are keen to help with immigration issues. You'll need to check-in/extensions and you'll be required to prove where you are staying. Hotels do that for you. AirBnB - not likely to help you. Maybe you can find an AirBnB host that will give you the paperwork to do it yourself.

Some people checkin to hotel few days before their immigration business, do their immigration business, go back to AirBNB.

I am planning to do the same..and have booked 3.5 months so far inclusive of ASQ..but taken out insurance for 12 months..and have flexibility with the return flights..The rest will be sorted out in country because there are unknown address changes at moment

You CANNOT apply for the STV inside Thailand... only at a Thai Embassy or Consulate usually in your home country.

I'm on an STV currently with my first extension due in April.

I only know one person so far whom has done their first extension. I don't recall their name on the FB groups but this is what was required, I saved it for when I will be needing to do this in April.

I spoke to someone at the Thai Embassy in DC and they said Airbnb will work to satisfy the accommodation requirements and to simply upload booking confirmation showing the address, confirmation number etc. I've not finished my application yet so can't say for sure if that actually works, but this is what I was told.

A condition of the STV is that it can be extended up to nine months, so unless you're a bad boy whilst you're here, there is no reason whatsoever that your stay will not be extended. You have to give your location for the first three months. It is possible (but not tested yet) that when you go for your first extension, you may be asked for your confirmed plans for the next three months, but you won't know that until the time comes. Of course, once you're in the country you can make a booking which has a cancellation clause (I'd never suggest you do that of course 🤣)