If you want the points from Mitts, the solution is very simple. Give him PP1 time. That'll add 15 points per 82 with ease. If he gets that, he's going to put up a career high in points. If he doesn't, he will hover roughly where he is at. Which is clearly 2nd line (if not better territory). This season Mitts finished 47th in the entire league including all positions in ES points (if you push that to listed centers he's 25th and at least 5 of those guys rarely, if ever, play center... IE Guentzel and Nyqvist are listed at center). He is still 7th in ES points in the playoffs. In the regular season some names he was ahead of: Makar, Suzuki, Keller, Tavares, Jarvis, Boldy, Eichel, Kopitar, Jack Hughes, JEE, and Hintz. Some guys he was tied with: Boeser, Horvat, and Nelson. Guys he was 3 or less points behind: Barkov, Duchene, and Stutzle.
If you don't think his offense is up to snuff, you're not paying attention or willingly blind because he was traded for Byram.