tried your suggestion. Works perfectly
I'm not sure any more. Most probably this attempt is better: https://github.com/ThomasKaise…/overlay/99raspberrypiorg (replacing the first two entries with 'buster' of course). IIRC back then with the old OMV3 attempt user installations ended up with a mix of Raspbian and Debian packages. But only those 5 packages firmware-brcm80211 libraspberrypi-bin libraspberrypi0 raspberrypi-bootloader raspberrypi-kernel should be fetched from archive.raspberrypi.org and everything else should come from upstream Debian repos...