Let's say I am a law-abiding German citizen, but I have 3 BTC, that have been swapped from XMR and I can't prove the origin of them at all. Then after holding for 1 year, I cash them out tax free to my private bank account via coinbase or something.

Can the fiscal authorities (Finanzamt) then force me to prove the origins of my BTC? What if I don't do that. Would I be doing a crime, even if I am not money laundering and there is no further evidence in that direction? What crime would that be? Could they take my money?