A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system - John Gall

Build the simplest thing you can think of first and then iterate on it. That doesn’t mean don’t think through parts of the system that are going to be complicated but until you start building you’ll not find out what works and what doesn’t.

If there are integration points with other pieces of software you can start them using the simple version and make sure all parties are on the same page. You can go as simple are hardcoding a response to start with and then replacing it with the working code when you have that ready.