FYI:
I bought a Edimax EW-7611UXB - 2-in-1 WLAN 6 AX900 & Bluetooth 5.3 USB-Adapter ( Amazon.de ).
Small, only one dongle and sufficiently fast and modern.
Unfortunately no driver support under Oram due to older kernel…
…which has been fixed by vangelis/trixie! 
After adding the following lines to the [all] section of /boot/firmware/config.txt
dtoverlay=disable-wifi
dtoverlay=disable-bt
and a reboot, the internal devices were deactivated and I could simply configure the USB devices via the regular zynthian menus.
Caveat: Have to change the /boot/firmware/config.txt after each update for having the internal devices deactivated… I might lock the file to prevent it from being updated in the future but don’t want to do this in the current stage of activity/fixing on vangelis for not missing potential relevant changes in this file…