I think him slaughtering Tusken Raiders is enough going dark for one long movie. It’s explaining what happened to Dooku that would be your problem.
Since Grievous’ll probably be cut anyways, replace him on Utapau with Dooku. Of course, you can’t just rotoscope the whole fight, so cut it. Instead, add Dooku’s solar sailer being shot down in [this] shot, with a reaction shot of him plummeting to his doom. Mix that in with the other shots of clones and droids fighting and you should be good.