What is the worst your gonna eat?
Literally? Tipper f-smash that's DIed badly because you tech rolled to the edge and held the direction (and subsequently died at like 30%). But that's also a huge gamble on his part since it's hard to do and if you do anything else his positioning isn't very good for it.
Small note: On YS at low percents if he's tossing me off I sometimes DI into him so he has to combo rather than chancing slope BS. When spike goes online and kills this obviously doesn't work but I hate chancing random tippers at stupid low percents on that level. It's not worth it IMO unless you're super confident you'll get it. 20% throw combo is manageable - dying at 20% is not.
That said, that's a situational exception to the rule. In general force him to tech chase you. Doiwn and away from either directional throw.
Marth's tech chases are very manageable because it builds damage slowly (regrabs work in segments of 4-7%?) and Peach has an awesome reversal if he's even slightly off. He lacks a good ground normal that he can launch with like Sheik f-tilt or Peach dash attack, so his conversions are limited. He often tries to jump and aerial to transition but this obviously gives you a clue as to how to tech to escape (if he dairs forward, rolling through him is great and so forth - he can obviously use this as a bait but most will want to try to start the combo and commit to an option or try to fade to cover 2 or 3).
If you DI in, he gets an air combo. Air combos are significantly less manageable because if he holds you at a good height and prevents you from going into the stratosphere by hobbling you carefully with strategic soft hits (namely fair) and purely horizontal moves (fair again, but nair is really underrated for comboing Peach) then you can get stuck in positions where your airborne reversals and escape options can be circumvented and countered really easily. Marth is cruel. Also, f-throw > tipped u-tilt is stupid at silly KO percents if you DI the throw in (but most Marths still f-smash for some reason).
MU is mostly won by getting under him and then covering his descent with various things. Push him to the edge and keep him there since his ledge recoveries are meh compared to Fox, Falco, and Sheik's and you can zone him / pressure with turnips really effectively there (he doesn't benefit from holding the veggie so him catching it and then throwing it at you to disrupt is also less of an issue). Keep him from linking throw > kill move at all costs. Turnips are good for zoning his dash dance and pressing him to the edge but don't try to follow every single one since you're still slow and your ground covers are meh vs his dash dance & shield-WD mix - he seriously doesn't need much stage to dance around you (a lot of Peaches overextend with fair and then jab or w/e and make a move that's hard to punish easy to). Also... if you can edgeguard him decently, huge plus to you in the MU because most Peaches are terrible at it. Jumping off with attacks and trading is super pro so long as you don't get spiked or something. Same with holding the edge with lots of invincibility to force him onstage and then standing for the free hit - do your favourite combo at low percents and run by FC nair / charge d-smash otherwise. Faking the turnip throw and then going for an aerial (float the aerial or z-drop the turnip to do this), or floating out to aerial while holding a turnip and then backing off when he commits to protect himself (this is also where you chuck the veggie at him) are really good simple tricks too for disrupting his returns.