I’m not in Germany, but in Switzerland which has a similar system.
As far as I know, the church tax is not made mandatory by the state per se, but it is an agreement in which the state puts its fund-collecting powers and structures to the service of the churches, which it recognizes as contributing to common good and public welfare.
In my own Swiss state, the churches have generously decided, half a century ago, to give up the mandatory character of the church tax (the state still helps collecting the taxes people willingly decide to pay). As a result, the better part of our energies is spent collecting funds and worrying about how we’re going to survive, since the number of people who contribute is in steady decline.
Getting off the church members’ list - and thus not being asked to pay the tax - is a very simple process, and of course, we’re still going to marry them, baptize their children, and bury them for free, so…