If you stay in Thailand for more than 180 days in a calendar year, you must file a tax return. If the tax rate in Germany is lower than in Thailand, you must pay the difference in Thailand.
If you are applying for a specific visa, you should know what you need it for. The DTV has been discussed hundreds of times in this forum and there is just as much information about it. So I think it is unnecessary to keep asking about something that we have already received an answer to hundreds of times. And no, I am not jealous because I cannot get one, I have been living in Thailand for many years on a different visa.
When I needed to open a bank account 8 years ago for the retirement visa, SCB and 3 other Thai banks refused to give me one. They all wanted to see a working permit, which you don't have as you're a pensioner. Finally I got a Kasikorn Bank account within a few minutes.