AD mentioned something similar to this a while back and it looks like the best option. Make a trade with Tampa for Callahan and one of their mid-priced wingers (Miller, Palat, Gourde) for Gustafsson + ??? 2019 2nd rounder or Kurashev. Tampa has a major salary crunch coming and they need to move a player to sign Point and will lose some dmen so they would have a need for Gus. Callahan's deal is for only 1 more season so it would not affect 'Cat and Strome deals.
I know you don't want to give up Gus but Jokiharju will be a full-time top 4 dman next year with others coming in future years. And getting one of those Tampa forwards would solidify your top 6 sliding down someone like Saad, Sikura, or Kahun to your 3rd thus strengthening your top 3 lines overall.
Another thought is Toronto. Gus at 1.2 would look really good for them if they are losing Gardiner. Johnsson or Kapanen would be ideal targets if they become available.