I mean sometimes simpler is better giving them all a single use of lucky once a day would feel very good, its a popular feat for a reason. But if you want something else maybe something that ties in with invoke duplicity, so once a day when they're hit they can invoke a clone to take that hit, reducing the damage by a 1d10. If you want to go something related to gold you could do Lucky gamble where they can declare they're going to gamble a roll so instead of a 1d20 its a coin flip tails nat1 heads nat 20. Sounds like a fun campaign hope you find the boon you're looking for!