They didnt work in Thailand. Previously to work online you still needed a work permit even if you had no Thai clients, with the DTV that is not the case.
Anonymous participant No.. You can be a freelancer just as easily on a DTV.
Also you dont have to do this in advance of getting the DTV, the visa and tax offices are not linked to check those elements its about you getting your tax affairs efficiently set up as possible over the long term.
You need professional advice.. It is likely far cheaper to sign out of germany, and become a non resident for taxation, and invoice for your work as an overseas contractor. This should break the connection and it stops being domestic source income in Germany. The self employment tests (multiple clients, responsibility for work etc) stop being an active test as you are no longer under German rules.
At that point you will only pay income tax on the funds you remit to Thailand.