Let me give a bit of support for your current plan. Why not start with a sign? "Want to get in on our expansion plans?" and then a QR code and an URL pointing to an e-mail harvesting landing page should let you gauge interest. Start a conversation with these people to find out if you have investors, franchisees, or even if there's any interest at all. This would be a cheap way to put a toe in the water and if nobody bites, franchising is probably not the way to go. Instead put money aside and hire employees to work a second cart when you have sufficient put aside.
You have a built in advertising platform for your expansion. That's better than a lot of businesses. Use it.