Heda Well, it's sadly not that easy. First of all: it's correct, that you don't have to pay taxes for every ⏠up to your personal limit.
But: the german system is more like a "opt-out"-system. Only the tax authority is in position to tell, if you're under your personal limit or not. Not the bank, not you, the final decision lies in the hand of the tax authority.
For every interest on your german bank the bank will take the tax automatically - even if you're below your limit. If you're below your limit you have to do a tax declaration. If the finance authority agrees, that you're below the limit, you will get your taxes back.
You can do a so called "Freistellungsauftrag". Thats - in a easy way - a tax declaration just for interests. You tell your bank ahead of the interests-payment that you want to declare x⏠as tax-free and the bank will do this. You don't need to do a tax declaration afterwards, because every Freistellungsauftrag will be transfered to the tax authority by your bank. So the tax authority is still in charge of the final decision about your taxes. If you, for example, tell two banks to do a Freistellungsauftrag, but the total amount is more than your limit, you definitively will get a call by the tax authority.
That means: as bunq don't do the automatic tax-thing AND you cannot give them a german Freistellungsauftrag, you must do a tax declaration, no matter how much interests you get. As the tax decision is by the tax authority and not by you, you must tell them every interest you get. Additionally to that you must announce the interests at every other bank in your tax declaration, too, because it's a tax declaration - your have to tell them anything in this report. Even if you did a Freistellungsauftrag.
At all, you won't have to pay for your interests, as long as they're below your limit, but it's clearly means (much) time to do the tax declaration. Additional to that: we still don't know how the interests get paid. If they will be added to every IBAN of your bunq-account, this will mean a lot of paperwork for your tax declaration.
I think the interests are fine and a nice bonus for bunq-users, but I'm not sure if the stress with your tax authority and the paperwork is the 27 âŹ/yearly worth it...