Meanwhile, I get four crimson keys about two weeks ago while trying to grind for frozen turtle shell.
Vampire Knives is far from the only viable choice for Cultist; it offers lifesteal but its damage won’t do much. In fact, it shouldn’t be your main at all - it’s used as a secondary to recover HP, not to deal damage. There are plenty of weapons you can use:
Influx Waver is the best sword to use here, since it travels in a straight lime and is easy to aim, and since Cultist is usually stationary it consistently hits thrice.
Flairon probably has the best airborne DPS of any weapons you can consistently get pre-cultist, although the bubbles get a slight penalty because Cultist takes reduced damage from homing.
Eye of Cthulhu is an easy weapon to use on this boss, again since he’s mostly stationary, and can pull great sustained damage versus a single target.
There are more, but those are probably the best choices.
As some side tips, when Cultist does the ritual attack, make sure to hit the real cultist. It’s easier to distinguish at day because the fakes are somewhat translucent. Orbiting the boss is usually the best way to avoid all the attacks, except for the ice mist, where you just travel with the ice ball itself. I recommend a UFO mount or witch’s broom in Expert, their specific form of maneuverability is really good in this fight.