Segregation has worked for me too. Turning off QoS on my AX-82U improved the situation, but I was still having a few of my 9 LIFX bulbs disconnect intermittently, as well as a few other IoT devices like my washing machine.
I've now pulled an old RT-N66U out of retirement - running LTS firmware...
I've been banging my head against a wall for the last week with this - LIFX bulbs keep disconnecting on my new RT-AX82U, they were all fine on my old R7000.
Turning off Adaptive QoS seems to have finally got it stable and working over the last 48h. I'm going to experiment with turning it back...