Welcome to the endgame of Granblue Fantasy Relink
Everything that you can do to min-max a character is locked behind some atrocious gacha roll, Curios being the main one.
The sigils (what I was looking through) don't drop directly in stages. Instead you can get UP TO A SINGLE CURIO per mission.
They can't be save scummed (like another gacha in this game) so all you can do is grind... grind...grind...grind hoping the curios that drop are decent.
There's nothing for players to work towards besides just mindlessly running missions to get curios.