If you’re just looking to integrate tRPC, my advice would be to scaffold an app with that, look at how we do it, and copy paste into your project until it works
I would say it depends on your level of customization in your nextjs app. Create-t3-app is nextjs based so in theory you can spin up a new create-t3-app and copy/paste your existing project files into the new project. There'd probably be a few files you'd need to merge rather than copy/paste like the _app.tsx file, but if you're just coping over pages and components you're good. After that you can start converting your /api endpoints to trpc one by one as you're ready.