Assuming it’s not an easily replicable app.

It takes a lot to go from zero to one, and finding paying customers and traction. First to market doesn’t always mean you’ll automatically win either. Defining the problem you solve with your audience is critical. Let the folks copying you help position you as the category leader. Marketing yourself to be larger than life makes you look like you have a solid defensible moat, and may cause them to reconsider direct competition.

Check out the book Play Bigger. It’s a quick read. Best of luck to you!

You haven’t signed anything. It’s not too late. It’ll be very very uncomfortable and you may lose the deal, but then you’re not stuck in a commitment and can move on. Perhaps find a middle ground. Time-box the deal for the first few months and set a mutual out and for renegotiation.