The problem is that the house is wired so the switching occurs at the legacy switch location. The device that does the actual power control has to be in the same circuit. So, it would have to be relocated to hide it.
If the other side of the wall (if it's an internal wall) is, say, a closet, then it could be moved there and not seen. Then, like your picture shows, your single gang remote can remotely tell the hidden switch to turn on / off / dim.
You could replace the bulb with a smart bulb so the actual switching is within the bulb itself. More expensive bulb, some minor rewiring, and some drywall patching. Note that where you take the wires off the current switch and connect them is supposed to be accessible (not covered in drywall) for safety. So, possibly put in the single gang box, which will get crowded quick.
Another way is to rerun the controlled wire to a remote location where you have RadioRa2 switches out of sight.
Doable, but it will be a project.