Ok I've never used Rails, but personally I've enjoyed my experience with a monorepo (pnpm workspaces) consisting of a Node.js API server and Next.js frontend server. You could try doing everything in Next.js, but I prefer keeping my backend separate. Love having everything in a monorepo though.

If you want more coupling, perhaps you could try something like ts-rest to add API types to your frontend. Or if you're feeling more adventurous, GraphQL or tRPC, though I personally prefer to avoid being tied to overly experimental frameworks.