Unfortunately I don't have any experience with getting this set up with App Router instead of Pages, but I may cover this in the future. It's still in beta, but maybe Webflow DevLink (https://webflow.com/devlink) could help, but it's in beta and it works very different than this.

I'm not entirely sure. I don't think I've ever tried to change canonical links with Cloudflare workers. They have lots of examples in their docs though. You should be able to modify one of those examples to suit your needs.