If you genuinely care about productivity go with Svelte. React is unproductive and unintutive.
On the other hand NextJS/React have been both productive and intuitive. There are so many examples out there and people are constantly talking about the nuances of both here and on other sites.
It'd be great to see Next integrate something like TRPC/OpenAPI into it so calls to the backend could be statically typed. That would really polish up the end to end experience.
I've been using their superjson library with Remix & its pretty much solved all of my client/server type issues.
I feel like StackOverflow for Svelte is healthy and due to the batteries-included approach for storage and CSS the community is a lot less fragmented than the React community was (and maybe still is, I don't know).