A quick web search later I fell over this repository with a guide on how to get Chromium’s WebHID connection to work to a Keychron Q5 HE under ArchLinux, and I figured that was close enough (and the steps outlined therein logical enough) to adapt. And just so I can easily find this again, I’m replicating the crucial bits here…

Apparently all I needed were some additional udev rules for both the mouse itself (when connected via the USB-C cable) as well as the 2.4GHz dongle (when connected wirelessly). So, first I figured out the relevant vendor and product IDs:

$ lsusb | grep -i keychron
Bus 005 Device 042: ID 3434:d028 Keychron Keychron Ultra-Link 8K
Bus 005 Device 062: ID 3434:d06f Keychron Keychron G5

Those turned out to be vendor 0x3434 and products 0xd06f (wired mouse) and 0xd028 (dongle). So based on the repo linked above I created /etc/udev/rules.d/50-keychron-g5.rules as follows:

# Keychron G5 (WebHID)
KERNEL=="hidraw*", SUBSYSTEM=="hidraw", ATTR{idVendor}="3434", ATTRS{idProduct}=="d06f", MODE="0666", TAG+="uaccess", TAG+="udev-acl"
KERNEL=="hidraw*", SUBSYSTEM=="hidraw", ATTR{idVendor}="3434", ATTRS{idProduct}=="d028", MODE="0666", TAG+="uaccess", TAG+="udev-acl"

then refreshed udev:

sudo udevadm control --reload-rules
sudo udevadm trigger

and replugged the mouse and restarted Chromium.

After that, the launcher page could finally connect! 🥳