Firstly, learn the MU. Every character's spacing tools are different. You should learn their properties, how they are frequently used, and when/why they are used in the first place.
My short, general answer is to stay mobile. The primary objective of spacing is to keep a player in a specific zone, don't let them do that. Foxtrotting and crouching/crawling can work well because you force your opponent to change their spacing patterns. Even walking can do the trick, especially if they're using retreating aerials.
Rosalina may not have the greatest approach game and zone breaking, but simply advancing into their space can pressure them. Be careful though. This is riskier because you're trading your own spacing and Luma's safety for better stage control and potential mistakes from your opponent.
As for Corrin, his F-Smash and Side B give me the most trouble. Any effort to keep Luma safe (on the ground at least) will be difficult because he can literally impale you and kick Luma away lol. Luma will take a beating, but Corrin has less mobility than Marth so moving around his zone isn't quite as difficult. However, you can still punish Corrin for unsafe moves on shield, just Luma may not always be around to help.
I'm going to watch more Rosa vids to see how people like to zone now lol.