What are the requirements for extending a DTV visa in Thailand and can I use a TM30 from a hotel for proof of residence?

You're going to have to exit and re-enter after a single in country extension anyways, so you might as well just start bouncing now. It'll likely take you days to gather the requirements and you'll be put under consideration and need to return to get your stamp. Or you can bounce all in a few hours.

Far too hard when you can simply visit a neighbouring country and return resetting your stay. Not sure why you would want to put yourself through this when there is just no need

Why would you do that just do a border bounce

As

@Bra****

says most immigration offices don’t want to do the extension. Adding to that for your part 3 question, Phuket requires at least 15 days left on your current stamp to even consider doing it. Other offices may vary.

after 180 days why not leave the country?

Most just exit the country and re-enter to get their next 180 day stamp currently without the need for any documentation.

I'd be willing to bet the number of people that have gotten an extension on DTV is under 100. Immigration offices DO NOT want to give these extensions. Many have made very difficult requirements to extend, and then when you meet them, they come up with a new one that you don't have the paperwork for. Then you go and get the new thing they asked for and when you get back to the office there's another new thing added. On and on until you give up and do a border bounce instead. Most of the immigration officers will straight up tell you to just do a border bounce.

As far as the specific requirements, each immigration office has their own. So no one can tell you something is or is not a requirement because there are over 70 provinces and each has their own immigration office. I know for example in Chiang Mai they require you to show 500,000 baht in a Thai bank account. But people on a DTV cannot open a bank account. Therefore most people would never be able to get an extension in Chiang Mai.