I don't know if it's coincidence or not, but I found that when opening a lot of coffers one after the other, I tend to get a lot of the same item. Maybe it's just rng paranoia, but you could try opening them with little breaks in between or on different characters.
before the collection book i always just tried to have at least 1 of each monster shoulder for use if i wanted it, i fill in the remainders with gold vendor when they pop up there
doing this method ive actually not spent keys on anything other than the undaunted outfit pages for almost 2 years and have large majority of monster shoulders collected
the gold vendor monster stuff is especially good now that it is a 1 time buy to unlock things instead of needing to repeat buy if you wanted on multiple characters
i have my main house (grand topal hideaway) listed in the housing tours, it has multiple target dummies, scribing altar, and grandmaster stations (in progress being filled out), as well as almost every antiquity furnishing on display to preview them