Melee Peach main and former Sheik main/secondary here
Sheik's nice, but I started to have diminishing returns. I picked up Fox a while back and the technical demands of him sort of bled over (i.e. crisper FC aerials OOS, movement, etc.) It always helps to remember that Armada, save for a short stint of Y. Link vs Jiggs, ran train over majors using Peach alone for a while. She has a high ceiling, so be careful that Sheik doesn't just become a cure-all for matchups you don't enjoy. I started to use Peach against Jiggs (after ironically picking up Fox for the matchup) and found it more natural to make up the deficit of the match up with my own aptitude.
It'll make you a better player overall.