First, Howard is, indeed, your best young player. Which in my opinion means he’s exactly the kind of player you keep in this rebuild. He’ll still be in his prime when the Dolphins are good again and once you get done replacing veteran talent, cap space won’t be a huge issue. Yes, he’ll cost money. But you are going to have to spend money and these are the kinds of players you want to invest it in.
I will add one caveat to the above. If the deal involves accumulating picks because you have targeted a quarterback you like, either in 2019 or 2020, then I would support the trade.