We went through a very similar migration. Had a Next.js landing page and a separate TanStack Router SPA - consolidated both into a single Vite + TanStack Start app. Same experience with build times and the architecture mismatch: our app is heavily client-side with real-time state, and fighting Next.js's server-first assumptions wasn't worth it. TanStack Router's type-safe routing and file-based route generation have been great.