Disclaimer: I am not a tax advisor or tax lawyer. All information provided here is my best knowledge but I can not be held accountable for it. Please talk to your tax lawyer for more information!
The short answer is: Yes! All rewards you receive from delegating/staking your ADA is a taxable event as soon as it exceeds €256.-/year exemption limit. Tax obligations occur the moment you receive the ADA to the corresponding price of ADA in €. So for example:
01.01.21: 150 ADA at a price of €1.00 each
06.01.21: 200 ADA at a price of €1.20 each
In the case above you have a taxable amount on €390.-
Staking rewards are taxed as a part of your progressive income tax. In theory this can be up to 45%.
No. The tax event happens as soon as you receive the rewards, no matter if you sell them or not. Selling the rewards at a later point is not taxable though. In case the price of ADA rises significantly you can cash out the speculation gains for free. In case the price of ADA decreases you still get taxed on the price at the point you received those ADA. It's your personal risk!
Instead of tracking your rewards manually you can use a tool provided by Adapools.org to calculate them by just providing a receive address of your wallet:
The most controversial discussion in the german crypto community is about the question whether the ADA you use for staking (not the rewards) can still be sold tax free after one year. In Germany private purchasing and selling of crypto is free of tax after one year of holding those coins. By participating in staking this so called "Haltefrist" is in question.
With the latest recommendation letter of the BMF of May 2022 the extension of the "Haltefrist" has been dropped. This said also the coins (and the gains by price fluctuation) used for staking can still be sold free of tax after one year!
The official letter in german can be found here!