If you have a TM30 you should be able to get a residence certificate. If you have a residence certificate and your thai phone number is registered against your passport you should be able to open a bank account
I don't see what's confusing? Do you have a job you can do remotely? Or do you plan to sign up for soft power and not work at all? If the latter can you show you have money to support yourself? That's just for you. Every member of your family would require their own DTV, and some embassies allow you to write a sponsorship letter for them but for such a large family I doubt any would allow it. You would also all need to leave Thailand every 180 days to get a new stamp. The DTV route seems totally unworkable to me. I don't know how you make the equivalent of 100k baht per month where you are, but even if you have that money guaranteed somehow you are still going to struggle to live on that in Thailand with absolutely nothing to cover emergencies such as hospital bills and so on
Even if you can get cover the 500k baht for a DTV for yourself what are the rest of them going to do. I can't see any consulate would allow you to sponsor 6 other people using that same 500k baht. According to Numbeo the average cost of living for a family of 4 in Bangkok is almost 80k baht per month before you pay rent. So that's 100k baht and you have 4 more mouths to feed on top of that. Its highly unlikely any of you will be able to get a job here and the 80 year old will struggle to get health insurance so there are also hospital bills to consider. This can give you an idea of costs and you can change the cities to view different areas, but there are also visa costs as well
Other than payslips you should get a letter from the company saying you have permission to work remotely from Thailand and that you won't work with any Thai entities or any clients inside Thailand
You can only apply using the Thai consulate where you are physically located. You can't pick and choose the consulate other than by travelling to the location of the consulate and then applying. You also have to remain in a location relevant to the consulate until the visa process completes