The tithe is tax deductible, but only up to 400€ per year. As for the "benefits", well, you can have a proper Christian burial, receive the sacraments, have a church wedding and participate in communion. That is for non-orthodox Christians Churches, and I have no experience with congregations of other faiths. All for the tiny price of 1% of your yearly base income before income tax (your yearly gross income after all prior deductions, i.e. social social security, tax free income and payments specifically exempted from income tax, like our variants of a 401k).
You have to calculate your tithe yourself, but if you don't, then the church will collect an estimate based on sex, age and education. Whether or not it is benefitial to let the church estimate your tithe depends on how much you earn above or below average. I have let the church estimate my tithe for years now, as they were generally way off the mark (to my benefit, obviously). How very Christian of me, huh?
That is the same, you can just visit any mass, funerals or baptisms, you are just not allowed to participate in communion. Theoretically, that is, it is not like anyone actually checks your congregation membership when you partake.
Data protection is just an excuse. Your parents sign you up via baptism, and at age fourteen you are supposed to confirm your membership through ritual, which I suppose is the same for the US offshoot of the Catholic Church, although I have to admit that I have not put in the effort to check. When you do, the moment you start earning an income that is not from any apprenticeship, the church will send you a letter, demanding money. The data your congregation has on you is simply your birthday, sex and place of residence.
How much this is just a silly hurdle designed to prevent people from readily leaving congregations they joined is seen in the inverse: if I were to sign up for another congregation, that can be done directly with them. Joining? Easy. Leaving? Oh no, please go to your district administration and tell them that you want to leave and they will inform your congregation for you. Just to make sure nothing goes wrong.
There is full separation between church and state here (in theory, at least), but the first couple of elected governments after World War 2 were lead by the People's Party (or, as it was called at the time, the Christian party) and had deeply Catholic roots, and thus decided to reinstate Austro-Hungarian Empire laws passed in 1868, but conveniently keeping the church tax law from Nazi Germany.