How much metal is there in use in the fixture? That may be distorting the signal that’s arriving at the bulb, or faraday cage. The rssi seems pretty good, is that what the mesh app is telling you? Or how did you get that reading?
There is a smart wall switch wish list item which is labeled as in development. I’ll link the thread so you can have some light reading if you would like. Also go ahead and set your thread notifications to “watching” so that you can be updated whenever the in development status changes.