That's the thing. In react, there's no difference between a route and a component (at least with react-router). So you can write logic that applies to a Route and logic that applies to a component the same way and apply it at the appropriate layer.

I don't know how Next.js' filesystem router works, but it seems to perhaps be the cause for this roundabout solution.

You can do middleware in client side routing with react-router similar to how you would with express. That's exactly what hooks are for and you can build your own to enable that functionality at the scope that's needed.