Just to let others know:
Meanwhile I tested 3 different sensors for light (lux) measurement via my ConBee II stick on Raspi 3B.
Hue Motion Indoor
Hue Motion Outdoor
Xiaomi Mija 0-83.000 Lux ZigBee 3.0 Smart Home-Lightsensor

The Hues do also measure light intensity continously and not only if motion is detected.
The Outdoor version worked quiet stable from the beginning on.
The stable connection for the Indoor version really runs me nuts.
The Xiaomi I was not able to connect to the ConBee at all.

A couple of days ago I updated the firmware on my ConBee stick which solved the issues with the Indoor Hue and also allows a connection of the Xiaomi device (not visible after connection in the Phoscon App, but in the deCONZ GUI). Both run now stable and report continously.

BUT: The firmware update on the stick (now Vers. 2.05.75, 264A0700) caused other, much more really significant problems in terms of the fact that at some point of time in the night the stick completely forgets his network, resp. nearly all devices in it.
I was lucky yet, that he did a re-connect of all of them after ~4hours, but on GitHub are cases metioned where users seems to have all devices newly learned via the Phoscon App.

So my hint: Wait for the next firmware version before updating your stick.