Since we probably wanting this for multiple destinations, what if we used a centralized way at our home powerswitch.
We could stack multiple boards, and just use one arduino nano and one nRF on top of it. We just have to care about link each stack to a different arduino output.
Based in this idea we could create a second pcb for stack mounting, this board will just have the SSR and a deep switch for easy selectable arduino output. To make this ok we should connect each home powerswitch to each stack, another way exist if we use the main supply of the bottom board for all stacks, and place our circuit before our home power switch, but this is not recommendable.
For minimum stack size SSR should be horizontal mounted.
The advantages are obvious, just one arduino and nRF, one ac-dc and regulator for all home switches. Super low volume occupied and no more size constraints inside wall switches.
I don't have time to go forward and design this, so if someone wants it please go for it!!
PS - Since we just use one arduino output per stack, we could avoid deep switch cost and configure by soldering selected track.