it makes sense. As no visa means: only 2 times land border crossing. With a visa you can cross as often as you want. In total a METV gives you close to 9month of stay. No visa, aka visa exempt + extension, gives you 8month. Unless you fly out and flay back in.
Yes, but only because you were to lazy to adjust 10 years ago. No idea why you prefer to pay taxes on pensions in your home country, while pensions are not taxed in Thailand ... you should have changed your official tax residency long ago.
The tax policy is not for foreigners. It is for Thai. And obviously foreigners are treated the same. You always were a tax resident in Thailand if you stayed more than 180 days per year. Regardless of visa or lack thereof (visa exempt).
Before the tax changes you could season your money earned outside of Thailand for a year and then bring it into Thailand, tax free. That is now no longer possible: for Thai, as well as for non Thai.
Then start googeling the paper work. It depends on from which countries you are both from, what paper work is required. German <-> Thai is a nightmare.
No, you have some things confused. I suggest to contemplate and reword your question to ask what you actually want to ask. Then delete this post. And ask the real question. If you think you can "abuse" the DTV as an monster ED visa: you read something wrong on the internet (and it would not make any sense as a real ED visa is cheaper).
ED visa do not have overcharges. They simply cost initially 4000THB. No idea where you get your info from. Visa vise the DTV is more expensive as it costs around 12,000THB and requires 500,000THB on a bank account.
You are confused about something. If you want one of them: the ED visa is most certainly the simplest and cheapest solution.
The DTV allows you to work, the ED visa does not ... any more questions?