I had many Zigbee problems after C5 -> C8 migration. I tried most of the recommended solutions. None worked. The end of my 30 day Amazon return window approached. I sent the C8 back and bought another one. Restoring my C8 setup on the new hardware solved all problems.
In effect, I did both a hard reset-restore and also changed the hardware. So I don't know whether it was the hard reset or the changed hardware that fixed all problems. My new setup has channel 20 and Zigbee power level 16 and works great. I never believed that lowering the power level made sense for a properly working system. I suspect that some people, including me, got Zigbee radio chips that are erratic or flawed in some small but essential way. But demonstrating that would require careful experiments, which I did not do.
For those with C8 problems, I particularly had difficulty with Hue Motion Sensor V2 (from Costco), Iris Motion sensor V3, and Iris Contact sensor. I have a very small and stable Zigbee network with two repeaters and six battery end devices. The various arguments about unstable Zigbee arrangements never made any sense for my setup.
