Hi , you can use armbian ( https://www.armbian.com/lamobo-r1/ ) , make sure is the version 5.20. Then you can use the Broadcom Swicth just like a PHY to provide just a single interface eth0. To ensure of that, better chek that
/etc/network/interfaces just contains:
# Wired adapter #1
iface eth0 inet dhcp
# Local loopback
iface lo inet loopback
With these you can easily achieve quite good speeds ( 600Mbps TX/ 900Mbps RX . Also , if you want to upgrade the distribution and you are in the vanilla kernel ( the one I tested ), you have to make sure that the upgrade is not on the kerne/u-boot packages ans from 5.21 the new vanilla kernel. To avoid that , create the a file
selections.txt with the following contents:
and then execute:
sudo dpkg --set-selections < selection.txt.
Hope that helps.