Hm...yeah that is a bit of a conundrum. I think people tend to assume patterns more often than not, so if there's one item that gives you more detail the more of them you collect, the player will assume there must be more. But since there's no way to tell from just looking at one of the item (or even multiples, neccessarily), and you don't want to miss out on any of the storylines, that paranoia just spreads to everything and leads to item hunting. (or it sorta did for me, anyway.)
So it might just be as simple as marking the items that you need multiples of? Not something as grotesquely obvious as a "1/20 TAPESTRY FRAGMENTS COLLECTED", obviously, but maybe just something subtle like a key phrase ("You wonder if there is something more to these...") or taking them to another screen when they examine the pieces. Then, as long as there are at least two examples, the player will be able to draw the connection and go "ah, ok, so that's how this works" and be reassured that they're not missing out by not hoarding 40 Slime-Covered Notes.
Granted, that does sort of gamify the experience in ways that aren't ideal, but I think that might just be unavoidable for any instance of "collect X objects to unlock Y story"? So it might be better to make that small sacrifice and confine it to an identifiable set of objects, rather than let that sort of item-paranoia leak out all over everything.
IDK, I'm just rambling now. You almost certainly know better than I do here.
No these are really interesting thoughts! And very similar to thoughts I've had myself about other mechanics.
Basically a lot of this design has been sort of trying furiously to swim against the whirlpool of typical game design, because like if you treat it like an RPG I think it gets pretty horribly frustrating? There's no way of grinding items, there's no kind of obvious predictable order to when certain things appear... and all that's intentional, but if any mechanic accidentally flips the behavioral switch in everyone's head now that says "I need to grind and hoard and grind some more because that's what you do in games" the whole house of cards kind of...
wait, I've mixed like three metaphors there, haven't I?
Whatever, the point is that like I tried aggressively to avoid gamifying this in the typical ways but part of that involves integrating in some of the useful design space that grindy games can offer while still finding ways to signal, as you suggest, that this is a kind of exception.
Actually, the jump to another screen might be the solution here, since it loops the tapestries in with the mask and the root, taking advantage of those existing mechanics...
I may give that a try! :D