I imagine the the chain wheels are 50/34 if so you likely won't have any issue with typical hills even if you had a 28 sprocket as your lowest gear. I'm not big on the Tiagra group set. The 105 5800 group set gives the most range of cassette options with 32 being the biggest. If you are a Speed demon and can pull a chain wheel bigger than 53, then Dura Ace would be what you should set your goals on. But I don't think you are quite there yet in leg strength. Few are unless you are built like Marcel Kittel. And I don't think he rides a ring bigger than that in Grand Tours.

The Domane is about to be four years old. That's way too much money. IMO, and these people here agree with me....Bicycle Blue Book

You can get a 2018 Trek Emonda ALR5 for the same price with a 105 group already on it. It's aluminum but it's light enough at 18.7 pounds.

Of course, this is my opinion with not enough knowledge of what kind of riding style you prefer and what your goals for biking are.