This is a suggestion: when salvaging treasure chest, give key fragment of for type of chest instead of gold. Later on combine key fragment into keys. Ratio is up to dev, my thoughts is 3 wood frag to 1 wood key, 4 iron frag to 1 iron key, 5 gold frag to 1 gold key etc…
Reason for this is once a player hit gold cap, run out keys and all chest slot used, there is no reason to play. At least this way player can still earn keys through gameplay.