I think there's likely a way to convert fiat to usdc but I don't think there's one where you can link a random user to an exchange without paying a fee (because they'd be providing the same service as you).

There's then the possibility of using systems already in defi like liquidity pools if your hypothetical exchange was considering allowing users to put up funds (on the chance the you didn't have the provisions for it). I'm assuming stripe has an api so you can confirm when a deposit has happened though.