What herbs [dried] should I use when making a tomato-based pasta sauce for prawns?

Well the spices won't make the prawns chewy, but overcooking them will. Just use some Italian spices and maybe add a it of crushed red pepper flakes. 

Last things first. When you cook the king prawns, use crab boil herbs in water, drop them in when the water is at high boil and take them off the heat when they become bright red. Cook them in the shell, and let them steep for several minutes to make them easier to peel.

Now about the sauce.

I believe you would like the finished dish better if you chose an Alfredo sauce or a Scampi. The Alfredo is creamy cheesy and the scampi is basically garlic butter. A caprese salad would go well with the shrimp, and serve with some crusty bread or rolls.

A little basil or oregano, or the 'Italian' mix that you have.

I'd saut'e the prawns lightly in a little butter, take them out of the pan, put to one side, then add back to the sauce at the end after the sauce has cooked, just to warm through before serving.

So they don't overcook and get tough.   

The Italian mix would be just fine to use. 

I would toss in some Red Pepper Flakes for a little spice. 

To prevent the prawns from getting chewy. 

You can either put them in the last few minutes of cooking (time will depend on their size) 

Or 

You can give them a nice sear on one die, remove them from the pan then finish the last few minutes in the sauce.  This will give you nice color, taste and texture on one side and not over cook them. 

Basil and oregano.