Hello! I like your idea! I can make you a system like this and in addition it will be fully customizable. For example, you said that you will have multiple restaurants using sub-domains. Well, from a brand's perspective, the menu should have a certain look, like the restaurant's logo and branding elements. Also, it would be a great idea to give them the possibility to make their menu page as awesome as they want, of they have the budget.
If you choose me, I am gonna make a system which will provide something similar to an API interface, so it would be very convenient to make new layouts and designs for this system. I am sure your future clients will appreciate this feature. It can even make the difference between success and failure.
If you are finding interesting what I had to say, send me a message and we'll discuss details.
Thank you, and good luck!