Zero suit might be a good pick. I've found that even bad ones can cause some problems for rosalina so if you get decent with her, you might have a better shot at reaching your goal.
The key to fighting ZSS is run --> short hop --> Nair.
Every ZSS I have played tries to stun Luma with the stun gun, but it's incredibly easy for Rosalina to punish it with short hop + Nair. You go over their stun gun, and you hit them. I know this sounds like elementary advice, but I've fought against clearly very skilled ZSS players and this works over and over again.
With ZSS it's just about getting used to her movements. She can seem dazzling if you're not used to where her attacks are going to move her, but once you get used to the patterns, ZSS's speed is not anything that Rosalina's range can't handle.
I say this as someone who has played 2500+ games with Rosalina now. I think a really good Yoshi player may in fact be Rosalina's worst match up.
Yoshi's eggs are really tough for Rosalina to deal with. Yes, Rosalina has gravitational pull, but that's a bad option when Rosalina is already close to Yoshi, and that's where the problem is.
Yoshi's eggs are really good at getting around Luma, and also good at stopping Rosalina's short hop + Nair approach.
Yoshi is also really good at taking advantage of one of Rosalina's few real weaknesses, her floatiness and getting back to the stage from the air. Yoshi's fast and powerful aerials really take advantage of this.
I honestly don't think Rosalina has any terrible match ups in this game. But right now Yoshi would be my pick for best counterpick.