The defense is going to go as far as McAvoy, Lindholm, and Carlo take it. There really isn’t any moves that they can realistically make to improve other than Lohrei taking a big step in Providence and getting called up for the playoffs. Every team going into the playoffs would like to add a “big, nasty, top 4 defenseman that can clear the crease and move the puck”, but if any of those guys are actually available, they are going to cost first round picks that the Bruins don’t have, and young roster players/prospects like Lohrei, Poitras, or Lysell.
They are much better off trying to upgrade through getting an upgrade in the top 6 assuming that they can clear the cap to make it happen IMO.