We'd like to use a fancy page for our collection page. For that, we can use the storefront API with our own server-side rendering app. However, is it possible for our website to use the Shopify theme except for the collection page that will use our custom storefront?
You will not able to load server side content from your own server app. Dynamic client side rendering is possible and you need to send a pageview when the contents has loaded. The content will be tracked by google then. More info here