Steel, overall I just think you should be a LOT more cautious in the Peach matchup. Her aerials, jabs, and tilts have priority over like everything you do, and she can nair out of "combos" that would work on other characters.
You wanna zone her with needles and make SURE your attacks will connect... meaning, you have to hit her at a time where she has some kind of lag after an attack, or grab her when she is shielding. Basically, you still need to be aggressive since she can overpower you if you don't, but you have to space and time all your attacks well.
Peach has a slight advantage, but you can overcome it if you just time it well. To me, this matchup is like... Peach throwing around a bunch of turnips and high priority attacks, and Sheik has to weave in and out with random fairs/bairs/nairs because those aerials are really fast and can actually find openings in Peach's priority.
On the ground, she is susceptible to needles, and when she's floating in the air, she is susceptible to timing aerials or DACUS.
She's hard to deal with if she's using low altitude float aerials or short hop aerials... all of her aerials have very little landing lag. So you might find yourself rolling away a lot if you're trying to be more cautious... but that's okay. The only bad part is that you might get cornered at the ledge a lot, so try to prevent that. Sheik has a good ledge game but it's still hard to get past Peach if she's edge guarding you since she has priority. She can even dair or dsmash you when you're trying to use your ledge game lol... but it's okay, just be watchful and use the best option to get up.
Light is right about nair out of shield
especially when Peach is using aerials to harass you. It's like "get outta here hoe."
Overall just play it safer, including with Zelda. If you play against Peach a lot more, I think it will become easier and easier to find where her open spots are, because they are there.