The question now is what exactly you have to submit to the Thai tax authorities if you stay longer than 180 days a year. According to my information, you have to fill in a tax return anyway even if you are not liable to pay tax. But maybe there are different rules now and a tax statement from Germany is sufficient. It would be a good idea to ask the relevant authorities for advice, especially before you extend your 180 days at the immigration office.
You may be right, but I got information that anyone staying in Thailand for more than 180 days is required to file a tax return, even if their income is already taxed in their home country and are therefore tax exempt in Thailand.
Information we get from the Thai authorities are sometime confusing and not up to date.