This is a valid criticism of SSR and the complex mental model it requires to implement effectively, even with Next.js. You need a firm understanding of the HTTP request lifecycle and the serialization boundary. For a novice programmer starting with frontend dev, I would not recommend starting with Next for this reason (create-react-app would be better).
But for an experienced backend developer, the mental model of Next is no problem, and the flexibility of SSR will feel liberating. After a few weeks you might even put down the Flask!
If you want to use SSR on your pages, I'd recommend sampling some Next projects (just pick one of the examples and get it running locally). You might like what you find. And for closer integration with Python, I'm sure there are options for server-side rendering of JSX that you could use, although they'll all ultimately need to either wrap a JS runtime or emulate JSX compilation in Python. So you may as well just skip the middleman and go to straight to react, whether via a framework or otherwise.
If you don't want a heavy framework, or you just want basic CSR for some richer pages, you can accomplish a lot with just `react` and `esbuild`. Even SSR is just a matter of calling `ReactDOMServer.renderToPipeableStream()` and doesn't need a framework like Next. They're just nice because they take care of all the problems you'd end up solving in a robust deployment anyway.