This is pretty interesting. Solid.js just released a big update for SolidStart, which is their fullstack metaframework like Next.js.
They split out the router logic to a separate package to be router agnostic, but you can still use server-side loader functions to prepare data for the route.
The clientOnly wrapper to declare some code as client side only looks really simple. Easier than putting 'use client' everywhere in Next.js modules.