I believe it's very far from that simple. Stateless people in Thailand have to go through huge hurdles to get citizenship, it's not as simple as just having a relative that already has it. Half of my spouse's family and many of their friends are still officially stateless without a Thai ID card because it's expensive and in some cases all but impossible to obtain it, despite their families living in Thailand for multiple generations.
And the fact that you already have a US passport means you're no longer stateless.
It's for anyone who can qualify for it. But if having a path to PR or citizenship, the ability to get a job in Thailand, or having a Thai bank account are important to you, then the DTV is not conducive to any of those.
I've never heard of anyone successfully extending (not renewing) a non-OX visa in order to get the second 5 years. I've read reports of people that tried, including bringing everything that immigration should require to do so. But they were told that it wasn't possible.
If you are going to try, the money must be in a Thai bank account, in liquid cash.
You need to go to your immigration office to ask for the specific requirements. I believe it's 3 million baht but you'll want to check for yourself what your office requires. Expect to be rejected though.
I'm assuming you're Australian. Most people just go back to Australia and get a new non-OX using money in their home bank account, since the one consulate in Australia is the only one in the world Thai allows that. So Australians are the only one who use this visa.