How do I build Web3 AT and FOR scale

I've been asked to work on a project that might use a Web3 stack that's more complex than anything I've ever done. Given mockups and functionality requirements, where should I start? I know this project will be huge but we also want to move fast, so should I focus on backend functionality first or second? And should I be documenting now or later? I've never built an app at this scale so I don't even know if I should start with a diagram or plan first. And since the proj could be so large, I want to make sure I follow practices that allow the app to scale up without being a mess. Any recommended reading, videos, or planning frameworks are welcome answers!

edit: Using Postgres, Prisma ORM, GQL + Typescript for backend and react-native plus next.js for frontend