I'd like to try making some mashed cauliflower to take the place of regular mashed potatoes.Since the potatoes add a lot of fat to my diet.... Does anyone have a suggestion for something that's tasty and neither too fat-laden nor bland?

You can obtain mash potato texture by boiling diced celeriac (half inch-20 minutes) which, you can mash and add to the mash cauliflower.
Jerusalem artichoke or sunchoke can also work.

Both celeriac and sunchokes have complex carbohydrates that are very beneficial and more healthy than potato starch....