Since your DSLAM is Broadcom (ATU-C System Vendor ID) I would recommend a draytek vigor 130 and would put this into bridge mode and then use the flashed fritzbox as a PPPoE so your not double NATed.
When i put a BT Homehub 5 on my parents line (same DSL chipset as FritzBox 7412) I lost about 7Mbps in downstream but only about 4Mbos via upstream but the daytek synced it back up to the normal speeds within a few days, however if this don't bother you then it might be a good idea to look at the Lantiq vrx200 xDSL firmware recommendation thread as there mentions of people using them on various German VDSL2 ISPs.