@David-TP 

Hi, thanks for the answer, and thanks for the post I hadn't found. It says the option activates only v3 IGMP snooping, so I suppose v1/v2 are already activated, which implies my problems won't be solved by activating IGMP snooping.

I can't remember if the option was available in former firmware. I upgraded to new firmware as it allows to program a nightly reboot, that I supposed would solve, partially at least, my problems (but it didn't).

I noticed the issues occur when an event triggers an automation involving devices attached to Aqara hub and devices that are not. For instance, I press a button (Aqara device) that will light on a Nanoleaf bulb (Homekit, via Thread) : 1 time out of 10, the bulb won't light on, that's pretty annoying.

In this thread someone says activating IGMP snooping would make "connection rock solid" (as for now, mine is sand-like).

So far I'm very disappointed with home automation : who accepts that a switch will light the bulbs... most of the time ?

I replaced the native TV box Wi-Fi with a set of 3 Deco M9 plus, which clearly improved things (response times, whole house coverage, etc.), but automations still don't work as they should.