OrphanCrow
However, whether the org collects that compulsory tax or not, the org, as a "public law" status religion, is now in the position to collect that tax and that means that they have the right to access income tax records of their members.
I'am a german citizen.
The church has no possibility, to access the income records of their members.
Look here:
https://www.evangelisch-in-westfalen.de/kirche/haeufige-fragen/kirchensteuer/faq-kirchensteuer/#c1931
"Erfährt die Kirche durch die Kirchensteuer die Höhe meines Einkommens?
Nein, denn die Finanzämter, die im Auftrag der Kirche die Kirchensteuer mittels Steuerbescheid festsetzen, überweisen die Kirchensteuer monatlich als Gesamtsumme an die steuerberechtigte Kirche. Die Kirchensteuerzahlerinnen und -zahler bleiben damit anonym."
Translation:
"Does church tax give the church the amount of my income?
No, because the tax offices, which determine the church tax by tax assessment on behalf of the church, transfer the church tax monthly as a total amount to the church entitled to tax. The church tax payers thus remain anonymous."
OrphanCrow
Legally, the org is in the position to collect tax on any capital gains that its members make.
Yes, but you can stop this with sending this form (called "Sperrvermerk §51a") to the tax office:
https://www.formulare-bfinv.de/ffw/form/display.do?%24context=1AAE7BFDAF82DAA2826D
I have done this in June of 2014 - here a snippet of the answer from the tax office:

This year I left the JW "church" - not with a letter or a conversation with the elders.
In Germany you can leave any registered religion (also the JWs) by visiting the Registry office or District Court and paying 30 Euro fee.
I have done this and have now an official paper, that I left the JW religion.
So the goverment statistic about how many people leave there church ist now increased by 1 person (me) for the registered JW religion.
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In the year 1999 the german branch printed a large and bulky book about the registration as religion in germany. The books name:
"Jehovas Zeugen - als Religionsgemeinschaft anerkannt"
Translation:
"Jehovah's Witnesses Recognized as a Religious Community"
LA-3-X (ist the watchtower literature code)