not sure what you think of this, but how about breaking up the whole celestial event?
what I mean is, once you defeat the cultist, he drops an item that lets you trigger the pillars instead of them automatically spawning and once all the pillars are dead, you get a celestal sigil instead of the moon lord starting to spawn.
then add a way to craft the pillar summon item as well, maybe with ectoplasm (to make sure the dungeon is open), beetle husks (to make sure the golem is dead) and whatever else you consider fitting.
that would make grinding fragments much easier plus accidentally killing the cultist would no longer be as annoying.
what I mean is, once you defeat the cultist, he drops an item that lets you trigger the pillars instead of them automatically spawning and once all the pillars are dead, you get a celestal sigil instead of the moon lord starting to spawn.
then add a way to craft the pillar summon item as well, maybe with ectoplasm (to make sure the dungeon is open), beetle husks (to make sure the golem is dead) and whatever else you consider fitting.
that would make grinding fragments much easier plus accidentally killing the cultist would no longer be as annoying.