The problem isn't onboarding. The problem is that without a pool of users, the site is worthless to businesses. The fundamental question you have to answer is -- assume you can friends and family a couple businesses, fine -- why does business 7 pay you money, or even do anything at all for you? Or business 8, or 9, or 10...
Small businesses are barraged with calls from vendors like you.
Well it's not that black and white. Can op make the application/business appealing as a single sided marketplace to begin with? Maybe. If there is 1 side that can find value without having the other side onboard then there you go. Doesn't mean it's going to be easy. Still hard, but just address the challenges as they come.
op, take a couple months to build your product (considering this is a CRUD-like app) and then face the distribution problem.
Small businesses are barraged with calls from vendors like you.