How To Make a Cloudflare app

HI! I need some guidance on how to create a (probably open-source) cloudflare app that will help website owners create paywalls/patreons/subscriptions.

What I need help with is how to remove part of a site’s HTML using the cloudflare app. All I can see from the documentation is how to inject js and change DNS.

I have not started developing anything and am open to suggestions - maybe there is a better, cross-platform way to intercept websites’ traffic.

Docs: Cloudflare Apps