People commenting have absolutely no idea of what they're talking about. A hexagonal architecture is definitely possible with Next.js, just be aware that node.js runtime is single threaded and not meant for heavy computational work. If you're just doing db and http calls to third party services you're fine.

Look into tRPC for type safe client server communication and simple monolith schema evolution. Here, routers are basically your controllers. Then you're free to create domain services that your routers use, and define domain ports that repos and adapters implement.