I might approach peach in a really different way then most of you guys and this is only based off friendlies but well see.
The main things i see as problems for sonic are: FC nair, FC dair, FC bair, Turnips, and ofcourse dsmash
So basically side-b is out of the question in this MU it just doesent work in any way shape or form from what ive seen, just to get that out of the way.
Float cancelling in general is an issue for sonic because his neutral relies heavily on punishing whiffs, peach essentially has no whiffs so how do we punish? Honestly, this is the part i cant answer lol
FC nairs are fast, very fast. Sonic has a huge problem dealing with constant hitboxes which is exactly what that provides peach and if she has a turnip in hand just forget about it lol. From my experience youre just gonna get hit by these once in awhile, they dont have alot of range to them though so you SHOULD be able to easily avoid them, should being the keyword.
FC dair is another move that allows peach to just have constant hitboxes out, this all depends on the height shes doing the dairs at though. If peach is doing them low then you can go over her head with a fair forcing a tech and then grabbing from that. If peach goes higher ive found that sonic can actually do a side-b underneath the dairs and JC into a bair, this was before i learned how to do a running power shield though which is much safer, i do that by running at the dair (Must hit full run so 11 frames after dash is input) you full press shield and then as youre going through the dair hitboxes just tap the direction you were moving on both sticks and youll make it behind her no problem, then just jump out of shield and bair (Or if you stop early wait in shield and SH nair as the dair ends)
FC bair is a wall but its not disjointed by a ton so if you space out fair/bair you should be fine
Turnips are fun, i LOVE turnips and any item MU for sonic honestly because sonic is sooooo fast that he can just catch all of these items there throwing (Thank you wavedash catching!) and using that incredible speed plus item in hand can AGT into an aerial to approach relatively well on peach. Theres alot of crazy stuff sonic can do with items like side-b and down-b stuff using AGT but i havent tested any of that only had the idea so far
Now the last and final wall standing in the way this MU, Dsmash, the world destroyer. this to me isnt even the worst thing in the MU especially compared to float cancels, I literally just SH fair ontop of her head and punish after it lands, in no way should you ever be getting caught by this when youre next to her unless she has frame advantage and even then sonics floaty hell only take like 24% max, but if you are next to her this move has a 5 frame start up and if youre within range for dsmash you should be in range to Jab or boostgrab and those moves a 2 and 3 frames respectively
and if worst comes to worst our dsmash when spaced clanks and cancels hers
thats about all i got for peach, uair is a god send vs floaties is all i have to say