Most people don't change unless they have to, and rarely even then. You'd have to make it so that they can't visit Reddit anymore.
Even on reddit itself, you can't get people to move from a sick community with hostile moderation to the preferred community. /r/Canada got taken over by /r/metacanada what feels like decades ago, and they turned it into a post modern bigoted classist hellhole, but it still ranks far above the "real" Canadian sub /r/OnGaurdForThee.
Maybe better not to compete with existing communities. Develop some anchor communities on Lemmy that are doing their own thing on topics that aren't well served on Reddit.
"Reddit is awful. How do we move that here?"