Sell your crypto currency for Euro and then you have a proof of the sale. After your meeting with the banker you sent your Euro balance from the exchange to your bank account.
Be careful with Fidor Bank - they have awful customer service and there core business is to serve German Residence without any SCHUFA checks - so exactly the opposite client then the average joe Crypto holder nowadays with 6 and 7 figure amounts here.
What you gusy were talking about is the "AWV-Meldepflich der Bundesbank": https://www.owlaw.com/reporting-obligations-for-foreign-payments-in-germany-z4-awv-meldepflicht/
Keep in mind that you can easily face starting from 7 figure amounts criminal investigations from german justice authorities - Germany is one of the countries that is going much further and is digging much deeper then looking if you just paid taxes on your money.