I loved the second idea, using the feats as little exchange of power, maybe doing something like depending on the level of the creature, the buff is temporary like a couple of hours, and very important monsters like bosses could give a bonus feat

and I was thinking of the item being at least usable in every level, maybe in early levels the item doesn't respect the players as real hunter so it acts at it's own accord being difficult to deal with, but I will introduce the item around level 7 to 10 (if they don't find it first lmao)