It's taken over year for my household of three cats to come to some sort of an approximated harmony.
Oni (8.5lb F6) still screams on occasion if Ziggy (13lb F2, and very leggy, so bigger appearing than 13lbs) or Jax (10lb fluffy mutt of a cat) chase her. But other times they all run around with no issues. Oni still sniffs Jax and then batts him pretty often, but, fortunately, he just ignores her aggression.
It's taken over a year for Oni to feel comfortable enough to curl up with us when another cat is near.
Are the cats eating together or in sight/smell of each other? I think that's helped socialization significantly. Jax has to be in a crate or another room because his other nickname is Hoover, and the crate did wonders early on to allow them to be close while still limiting interaction during meals.
I agree with WW's comments, with the further observation that it takes time.
And I agree with you, Laura, finding the balance of allowing interaction without damage to anyone can be hard. You're doing great.