The AVM Fritz!Box 7412 is known to be particular bad for modem use, especially above 50 MBit/s, apparently due to filtering issues on the analogue side of it, replacing it does make (a lot of-) sense.
Personally I've used a Draytek Vigor 130 on a DTAG line (with IPv6 and everything) between 2017 and 2021, worked really well - the modern -super-vectoring/ profile 35 capable- successor of that would be the Vigor 167, which should work just the same. If you don't have super-vectoring enabled in your neighbourhood, there'd be plenty of cheap(er) lantiq VRX2x8 second hand options available for a song (including a couple of Broadcom ones without OpenWrt support, but which would still do as a mere external modem); moeller0 also knows a cheap entry level modem from DTAG (which I always forget about).
DTAG is heavily pushing for passwordless PPPoE (and Kundencenter) access, even if you do disable that, it will get re-enabled at the next opportunity, which means you can enter just about anything for username and password (if enabled). The config snippet below has been in service for me until I switched ISPs (to fibre), it's not obfuscated or censored, it really worked with those fake access credentials:
config interface 'wan'
option proto 'pppoe'
option ipv6 '1'
option device 'eth0.7'
option username 'user@t-online.de'
option password 'pass'
config interface 'wan6'
option device '@wan'
option proto 'dhcpv6'
(bthub5 as router (pre-DSA), albeit using a vigor 130 as modem and not the bthub5's internal one, via its (ethernet-) WAN port; no this setup didn't make sense, it was a temporary one for the overlap between VDSL and ftth, exploiting existing cabling and leaving the vigor 130 where it was).