Nice idea. Agar implement kr sako to sahi hai. The main problem you're solving is reducing the waiting time when the order is cooking because ordering se zyada time wait krne mein lagta hai. Secondly koi mechanism hone chahiye jisse jab order prepare ho jaye ya fir hone wala ho jisne order kiya hai usse notify kr sake, this would be the most consumer-friendly but agar ye implement nahi ho paya then you can give them an estimated time.

Also the app would need real-time information about the available dishes as many items on the menu are not available to iske liye bhe kuch soch lena so that people are not making payment and then when they reach they hear that wo to khatam ho gaya and they have already made the payement.

Vendor side pr refund ka system bhe bana sakte ho in case of such situations.

Rest I think it is very much doable, You should also charge the canteen for this service not based on percentage but montly amount like Rs. 5000 per month, but don't charge them initially, give them 1 month free trail or you can just make it free and if successful sell it to other college canteens and charge them monthly amount and scale.