You can make a French cleat as strong as you want. If you're moving the TV around a lot and having it that far away from the wall, I would be worried about the French cleat camming out if you don't make the plywood panel tall enough. You could mitigate this by clamping the cleat together with screws or actual clamps, but if I was going to be moving the TV around, I'd do a more rigid mounting system. I'd still use a plywood interface, and I'd still space it off the wall a bit to allow for cable routing, bolts for the TV mount, etc., but I would just bolt the whole thing to the wall.