Hi James,
You’re right. I was already there, at the modules, but started to name it package.
I have searched and read.
Linux kernel modules in Debian: Modules - Debian Wiki
If you want to get a module to autoload when a device is accessed you can often add lines to /etc/modules.
b53_mdio Control the hardware - this is part of the kernel for many years
https://cateee.net/lkddb/web-lkddb/B53_MDIO_DRIVER.html
dsa_core Configure the network - this is part of the kernel for many years
https://cateee.net/lkddb/web-lkddb/NET_DSA.html
The complete error message has a hint to two files:
[ 6.139744] bcm53xx stmmac-0:1e: failed to register switch: -517 /scripts/local-bottom/mdadm: 2: /scripts/local-bottom/mdadm: rm: not found
So I searched in Buster where they are and what is inside:
./usr/share/initramfs-tools/scripts/local-bottom/mdadm
./usr/share/initramfs-tools/scripts/local-block/mdadm
They are there and they look the same in Bullseye.
Looking at the modules with the same command:
root@freedombox:/# lsmod | grep 'rtl\|b53\|dsa'
this entry is missing in Bullseye:
Module Size Used by
devlink 57344 1 dsa_core
or is this module loaded, whether or not the IP-Connection is up?
…
I tried hard for a while, than I gave up, because of missing knowledge how to solve this problem.
Now I had the idea to search at the source, DEBIAN. And it sounds like the problem is known for a while:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=986767
Comments/Problems:
Installer can’t properly bring up BCM53125 with DSA - missing DSA module tag_brcm. Also, no USB is detected.
After replacing kernel modules in installer initrd.gz from linux-image-5.10.0-5-armmp_5.10.26-1_armhf.deb
DSA properly detect and configure Broadcom BCM53125 switch, but unable to use it. Installer
try enable individual interface (wan) and disable all other, but for DSA master
interface (eth0) MUST be enabled before wan interface and stay up.
After connect USB network card - installer is able to detects network and complete the installation.
The question is, what is the approach to get this solved on DEBIAN. Any ideas or pointers are welcome, I want to get my FreedomBox 
I will also join a call to talk about it, if the dialog makes it easier.