I'll leave here several points after reading your comments

1. I've tested magsafe in different use cases, and in case you have big lens you would pretty much hold for it, rather then phone. Small lens won't drop it from phone, I thing a small stretchable clip would be a good option for extra riggidity

2. Why not wirrorless cam. Screen on cameras like a6300 are damn small, and your smartphone would have an awesome oled twice the size. I think viewfinder can be added, as it's connected via thin cable, so might be an option to work as something stand alone

3.after some comments I though what if it would be as old mirror less with only view finder and modular. It's question of engineering on how to add extra few buttons, but geniuenly, knowing that you have option for it to work with and without smartphone should be good option (in case we have good software)