Best Approach for Building a SaaS with Web and Mobile Front-Ends?

With a monolithic architecture, be aware that the traffic limits can be lower compared to microservices. You might find that your app can only handle about 500 to 2000 concurrent users. For a mobile and web app setup, it's best to create a Laravel REST API that both platforms can use. Your web front-end would serve as a single-page app, and while I’m not super familiar with mobile development, this is typically how you’d set it up.

From my professional experience, using React and React Native is a solid choice. However, keep in mind that you won't be able to reuse all your code—the UI components will differ since React Native doesn’t use HTML elements (think View instead of div). That said, if you organize your code well, you could potentially reuse up to 80% of your business logic, APIs, and other shared aspects, but that can vary depending on your app's structure.

I manage a monorepo that includes a Fastify backend and both a Next.js app and an Expo app. This setup allows for great code reusability, so I can definitely recommend going that route. It can make your life easier when developing separate but related applications like yours.

A well-designed monolith can actually handle just as many requests as microservices. It all comes down to how you structure everything. Using a load balancer for horizontal scaling can work wonders. The choice between monolithic or microservices should really be based on your specific use case.