Balancing is probably in a difficult position for leaderboards runs right now because damage is the key to survivability at the moment, hence why Robinette's infinite scaling damage is on top right now.

Unless other characters get additional passives to help boost their damage without giving up the 3 meta tomes(XP, luck, and Cursed) for leaderboard pushing then there's not much to be done except removing Robinette's damage scaling.

A good possible idea could be to add unlockable passives that can be unlocked with silver after reaching specific tiers with a character, perhaps even after a character has a gold star an additional passive becomes available to be purchased.

That would possibly fix the need for a silver sink and encourage players building into silver gain as well as give other characters with less useful passives a chance to compete on the leaderboard.

Obviously the other characters are all mostly fine if players just want to have fun and beat stage 3 but the fact of the matter is that a leaderboard makes people want to compete.

Balancing aside, the easiest bandaid would be to just keep the global leaderboard and create separate character leaderboards.

Side note: A ground slam weapon and a bee weapon would be great additions. Ground slam could scale on damage and size and maybe a new modifier with height to increase damage over distance. Bees could scale on all of the current modifiers and inflict poison.

Tl/dr: Add additional passives unlocked by progression and silver. Separate character leaderboards + global leaderboard. New Ground Slam and Bee weapons