If you don't have proof of payment received for recent work/contracts you're probably better off just going through soft power. The restrictions on the visa are the same and you could still do remote work later when your business is active again.
The 90 day report isn't specific to the DTV its for all visas that you will be in the country more then 90 days on a single entry stamp, like a retirement visa. It's standard compliance hence why it wasn't listed solely under the DTV.
It's a 6 month multi entry tourist visa. You will still only receive 60 day stamps that can be extended to 90 days and require leaving the country atleast 1 time to use the full 6 months.
I used a thai bank account. The only additional questions where proof of where those funds came from outside thailand. I had to show transfers from us accounts etc to prove I wasn't earning money in thailand
They want to know how you make money. How did you get the 500k to start with? How do you plan to earn money going forward it's that simple. Seems like a reasonable question. If your answer is I don't have a plan and I only have 500k baht they will most likely denie you. They assume that you'll end up working illegally in thailand if you don't have a previous source of income to fall back on.