Custom Linux on Raumfeld Connector (Embedded linux audio multiroom device)

Hi,

I have a multiroom audio system called "raumfeld" from teufel.
I do have this system for about 10 years and unfortunetally it never got to a point that satisfied me.
Now i want to exchange the existing linux on this devices (some ARMv6, and some are ARMv7) with a custom one to make it a roon audio device.

The devices have some LED's, buttons and usb connections and you can update/reset the firmwarevia image if you put in a USB device with a proper firmware.
The images of raumfeld are signed, so that means i cant change it?!

So now my goal is to get a custom linux onto this board which does also allow firmware updates via USB and that it would be possible to restore the original firmware.

The big problem here is i do not really know much about how i can achieve this. Some times ago i did manage to build a custom linux with buildroot for a RPI device and to compile some C++ apps on that device, that was the "biggest thing" that i did in linux environment. (I am Software Developer and the only thing i did with linux was to compile my c++ apps und try them there)

So do you have some advice for me how to start this project?
First step would be to check how i can update the firmware/linux system to a custom one and to be able to restore the original firmware. But i have no clue how to start.
Does a "signed image" mean i can only install this image to that device?

Adding the other hardware to the kernel (eg. dac, leds, buttons,..) would be the next step

Thanks
Chris