Did you try if there’s enough room in the box behind the light fixture? I don’t know how the situation is in your country, but where I live, those are usually bigger boxes.
Yes, had a friend here who is electrician and we tried hard, but there simply is not enough space for the SONOFF device. And a fitting device would need to be much smaller. Reason is, that there is not only the switch in the box, but also cables for power plugs in the room(s). So the box is not small, but very / too full already.
Ah. That’s a pity! The ZBMiniR2 already is the smallest device I know of. Well, maybe those new Shelly mini devices (that also have Zigbee, but are not available yet) might be a bit smaller?
Do you have a link to the Shelly stuff you mention? Perhaps it is worth waiting for it.
And it actually does not need to be Zigbee in the scenario. Anything that will talk with home assistant would do.
Best option for me would be something that was integrated into an actual wall switch, but I cannot seem to find anything… I could even set the bulbs on always power on and just have a battery powered “blind switch” which replaces my current wall switch. It just must not take a lot of space 
The Shelly 1 mini is their smallest device that’s currently available. Last week they announced Gen4; those devices will also have Zigbee connectivity. Gen4 will have mini devices, I assume their size will be roughly the same as the Gen3 minis.
Thanks, they are indeed a bit smaller, but I really doubt that it’s enough and that they will fit.
Thanks a whole lot for your help!
I would have suggested a smart zigbee switch earlier and saved you the hassle, but since you mentioned you live in a rented place I figured you couldn’t / wouldn’t change the switches.
Yes, there are smart zigbee switches which can control smart bulbs. They key word to look for when searching here is “decoupled”. Basically, most Aqara switches support decoupled mode, but you can get more info here
Ah, thanks a lot. Yes, initially I did not want to change much, but when I have to fiddle with the switches anyhow…
Looks like the Light Switch H2 EU - Aqara could work. I just couldn’t find any information if the “decoupled” mode (they call it MARS, if I understand correctly) will work without their Hub and their app, so only with the HA Z2M integration… Do you know if this will work? I assume so, as Aqara is supposed to “work with home assistant”, but again could not find any hint about that in their webpage…