Specifically for Chiang Mai, someone else reported they require proof of 500k in a Thai bank account. Yes they know DTV people cannot get a bank account. That's the entire point though, they want to make it impossible for you to get an extension so you do a border bounce.
I see a drive to Chiang Khong in your near future.
The only "benefit" to using this service is the appointment. But you can make an appointment for free, so there's not actually any benefit to using this service. And you get to pay extra for it too.
So you submit all the required documents online. Then you must bring all of those documents with you to the immigration to get your extension stamp. It will be a 30 day tourism extension, it will not be 60 days, or any other number than 30. All tourism extensions are 30 days.
It's what I would do. If you arrive visa exempt there is nothing to do ahead of time except filling out your TDAC.
The odds of immigration asking to see proof of onward travel are very low. It's more likely the airline would ask for that before letting you check-in. But they can ask for that on a tourist visa as well, since both visa exempt and tourist visa are both tourist entries. (Though to be fair, the odds are lower if you have a tourist visa).
there's a specific list of things they're looking for. 5 or 6 of them. They are all disqualifying. The form is on the embassy website.
But most people don't want that visa. If you start with a non-OA and intend on moving to Thailand permanently, you're going to have to purchase the mandatory insurance every year, forever. And it's only going to get more and more expensive as you get older. There's absolutely zero advantage to getting a non-OA visa if you're planning on moving to Thailand. You should be getting a non-O if that's the case.