Best advice I could give would be to build a properly framed front wall on the tub surround to prevent deflection in the first place.

In the photo below, the top plate of the front wall is two 2x4s on edge supported by four vertical 2x4s. While the objective for the apron shown is different than yours, it's plenty sturdy. But you can modify as needed.

It's much better to eliminate structural movement for natural stone than to try to modify the stone to accommodate the movement of the structure that supports it.

Thank you Mongoct for your advice & input! Your picture helped me with the visulaization I need to talk to my contractor with confidence. We can actually make this work. Again, thank you!