You'd be amazed what slick tyres can do. Last year's race around the Netherlands gravel edition was won on 28mm slicks. Which considering how many other riders were running 50mm+ rubber for the sandy gravelly conditions is damn good going.
You'd be amazed how little traction you can get on the rear wheel of a lightly loaded tadpole trike. If there's any mud or wet brick or similar, a slick rear tyre will severely restrict your climbing ability. It's not like cycleman is going to get off and push.
Our foray into Kojaks on barakta's trike (before she went electric) concluded that for purely road riding, there's a decent amount of rolling resistance to be saved compared to Marathons. (And there are better slicks than Kojaks; their main advantage is that they're cheap.) They seemed to have a sweet spot for pressure in terms of comfort and handling, so some experimentation required.