- hire an accountant, 100EUR/month and you don't have to care about taxes that much, unless you go wildly profitable
- LLC/GmbH is a better idea than having your life on the line if SHTF (though corporate veil might require at least 2 partners in some countries/states)
- Estonia doesn't seem very recommended lately (the only easy part is to get e-citizenship but the rest seems to be overly complicated). UG in Germany might be what you need, but your taxes will still be around 50%. Did you think about moving to Switzerland/Luxembourg instead?
- think about having a company in Delaware for flexibility reasons. International taxes are more complicated though and Germany would still be your tax domicile, and demanding its share of your profits while bombarding you with unending bureaucracy, often years after