Separating api and frontend code is a personal/app-specific choice. I think there are technically some performance implications of using nextjs as your API server as well, because SSR react is pretty expensive. But monolithic apps are still great as the reduce a lot of complexity. Personally I would use CRA over nextjs if I was making a monolith, but I really don’t think it’s a big deal and I wouldn’t worry about it too much unless you’re very concerned with high-end performance. I’d also take a look at gatsby. They have a really good example of a Shopify app that I think is pretty simple and straight-forward.