I've never tried stealing the casket from Zarys, but if you have access to the Guidance cantrip (say from Shadowheart or the amulet available) then you can get an additional 1d4 on the next skill check, or if you have a bard with bardic inspiration they can give another party member a d6 to use on an ability check (or saving throw or attack roll). I think there's also a ring somewhere that offers a bonus to sleight of hand, but would need to rummage around to find details (EDIT: here are
details of the ring).
Oh, and the Cat's Grace feature of the Enhance Ability spell an give you advantage on dex skill checks including sleight of hand, but obviously that's only useful if a roll of 30 is possible in the first place.
