Yes, the addition of the "royal" to Sussex has always been the sticking point for me, but we'll see how that plays out. The Sussexes certainly have gone to a lot of trouble to set up the Sussex Royal brand, so using it may be non-negotiable for them. It all depends on how scorched earth they are willing to go.
Scorched earth policy would be totally stupid and really underscore and reinforce public perceptions that they are spitting in the face of the monarchy. It would make for a riveting interview but of little long term value to their lives.