Use Gloomhaven Helper to the max and move the components it replaced (a ton of them!!) in a box that stay in the shelf.

Separate things which are compatible between multiple games (class boxes, monsters, overlay tiles), and leave the game-specific content like items, events, and booklets, rules, etc separately.

These Ultra-Pro deck boxes fir all the class box contents and the mini (or two) but only if you sleeve only the number cards needed for each class to play (ie brute gets 10 sleeves, scoundrel gets 9 and players swap the cards in the sleeve if they want to make a pre-acebario pool swap. These ones you can add the attack modifier deck.