Zigbee Sonoff relays paired to one of their coordinators flashed with Tasmota. Although if the walls are too thick or tge sistances too high it could pose an issue for 2.4Ghz.
I have actually looked at Sonoff, but they won't fit into the wall. In order to fit into classic Danish design tradition our wall mounts/boxes are slightly to small to fit anything not specifically design for them. Our switches and socket looks a lot nice than anything I've seen from the US, EU or UK, but they are special little snowflakes.
There's an entire cottage industry and hobby scene around making 3D print and modifications to Ikea, Hue and other switches so that they'll look like "the real thing".
Even the mini modules? Maybe put them in the wall dose above the switch? Shellys are even larger. Sonoff seems to put out more miniaturized models every year.
