The dropping off of the ConBee and ConBee II devices is not uncommon. They are very susceptible to interference and also any voltage drop on the USB port.
The best thing I found with deConz and a Conbee or Conbee II.
Make sure it is no where near any other wireless or computer device. This is not just a Raspberry thing, if you plug it directly into a PC USB port without an extension cable it will sometimes fail to form a mesh, or get so much interference it doesn’t work
Use a powered USB hub so the USB port is powered independently of the device the Conbee is connected to.
By following the above, I got my conbee to work 99% of the time, though I still have times when it will fail to initialise after a full OS / container restart due to dropping off and not initialising before the rest of the system comes back up, needing Home Assistant to be restarted a couple of times before the USB is detected again.
If I was doing this again, I would not use deConz or a Conbee stick of any kind. They are too sensitive to interference and voltage changes on the USB line. I would get an Elelabs USB stick, a Digi Xbee USB stick or the good old TI CC2530 / CC2531 based dongle and use Zigbee2MQTT or the native ZHA integration.
Even now, with using a powered hub and keeping the Conbee II I have away from other devices (tried with 3 different Conbee II’s with same result btw) I would never trust it enough to act as any kind of alerting system while I was out or anything.