Speed to market, simplicity, and synchronicity between team tool familiarity cannot be understated in value for startup stack decisions. Thus, this is fast stack decision of infinite indecision.
The odds any member of your development team is not familiar with react(NextJS), python(FastAPI), and JSON(MongoDB) are next to nil. These frameworks are the latest stable breeds of fast to market and flexible implementations of some of the worlds most popular technologies. Better yet, they can easily be pushed to cloud and virtually infinitely scaled via serverless services simply using Vercel, Github, MongoDB Atlas, and a domain name provider. None of this even requires a credit card to start.
Start with Next: NextJS(aka next.js) brings the best of react bundled in a standardized Serverside rendering, ideal for webpage load speed optimization and the coresponding relationship to SEO ranking. NextJS is the most popular react framework and is appropriate for general web apps which meet most needs. In areas where NextJS does not meet needs, such as bundled mobile applications(realm of ReactNative), desktop applications(realm of Electron and Tauri), or SPA’s(traditional react); it is still React wrapper. So running clients of all of the examples above will reuse the majority of the code developed in NextJS.
The reality of this scenario, however, is these applications will universally require a standard webpage for information, even if only required by the application distribution platform like the Iphone, Google, and Windows app stores.