Riffing off your observation that it would be nice to have a charm whose traits were not negated by the carrying of a grim (in the same way that barkskin and natural bond can still help someone carrying a tome), I’d like to advance a handful of possible traits for both charms and trinkets… I think necklaces are pretty on point.
Charms:
What if proxy were inverted? What if the trait prevents you from using a potion, but always grants you the benefits of a potion that was consumed near you.
What if curse resistance was inverted? You have a trait that suppresses 33% of the curse of any grim you are carrying (thus protecting the party). This would be interesting because it would open up a property that has been historically locked down.
Going in a new direction, what about a charm trait that reduced the amount of damage you took when you were knocked down; this would be actively useful for grim carriers, or anyone for that matter.
Trinkets:
On the trinket side, it might be interesting to have a trait specifically for boss/monster hunting with bombs: increased power, decreased area, increased stagger on a direct hit.
Trinkets and Charms:
I think “substitution” might be an interesting trait for both trinkets and charms.
The trinket trait, “substitution” would allow you to carry an additional potion in place of your bomb;
the charm trait, “substitution” would allow you to carry an additional bomb in place of your potion.