@handcoding
Thank you for your message and the advice about my Zigbee network.
I made the requested changes to the modified script.
The strange thing is that I have repeaters all over the house. Besides this IKEA light bulb (which acts as a repeater since it’s permanently powered), I have about a dozen smart plugs with energy monitoring spread throughout the house, and they all act as repeaters as well. The only end devices I have are two remotes: one in the bedroom next to the IKEA light bulb, and one in my study next to the ConBee stick.
So I think this setup should be fairly stable, which makes me think I was perhaps a bit harsh in saying that the whole Zigbee network is unstable. 
The LQI from the two repeaters connected to the IKEA light bulb is around 90 (the signals have to pass through concrete), so maybe I do need to add another repeater on the second floor. 
The IKEA light bulb only fails every now and then—maybe once or twice a month. The rest of the time it seems to work just fine.