I managed to bridge the 2 Broadcom devices together using VLAN 35 (Canada).
The rpi3's lan port is connected to the LAN/WAN port of the TDW9970.
The TDW9970 is set to bridged VDSL mode on the Wan service page.
rpi3 sqm:
pppoe-wan35 24576/10240 kbits/s, cake/pieceofcake.qos, VDSL 44 byte PPO
23.08/9.5 Mbps d/u, A+/A+ bb/quality
40/47/42 msec max bb
40/42/41 msec avg bb
Streams 8 : 3
Link DSL sync 26489+ sync 10903+
Device / OS desktop / Windows Win10
Browser Chrome version: 91.0.4472.114
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