I don’t know the specifics but the OP chose a complicated setup. Why not just use a GmbH which is the equivalent to an Inc in the US? The capital requirement is 12500 in the bank not 25000 as stated. There are online services that run the whole process for you similar to Stripe Atlas. Even setting up a UG (with 1 euro stating capital) is very easy but you have to upgrade to a GmbH once you receive investment (investors usually demand it). For the whole process there are standard forms. You only need to pay lawyers if you want to personalize your setup.
Maybe OP was just not advised well which is surprising given the amount of information available online (startup guides for Germany)
I guess that’s what happens when you get advised by a lawyer.
They state that in the post, it is a tax-optimisation since it is a single-person company.
This. Is founded (and liquidated) my GmbH with only 12.500€ deposited to the business bank account. Note this is not a cost, it just moves to the corporate account and can be used to pay expenses (notary, lawyers, hardware etc.). The remaining 12.500 are a permanent loan that the proprietory needs to cover on in case there is a liquidity shortage or liability suit etc. In my case I never paid the remaining 12.500€, and in liquidation process it balances out to zero.
But if you think founding a german company is difficult, wait until have you do an ordered liquidation (not bankruptcy) - that is madness on its own.