I'm the author of HTTP Toolkit, I actually built the internals much earlier as an open-source library (Mockttp: https://github.com/httptoolkit/mockttp) designed for exactly the end-to-end testing mocking use case you're talking about.

It's MIT-licensed, and you can build an automated HTTP/HTTPS rewriting proxy using that in a handful of lines of JS, and script any kind of transformations or inject any responses you like.

There's a general guide to getting started here: https://httptoolkit.tech/blog/javascript-mitm-proxy-mockttp/.

For the more general interactive testing/debugging case, you can also use HTTP Toolkit itself (it has a rules builder for this kind of thing) but if you're building automation you should just use the internals directly, they have exactly the same capabilities. HTTP Toolkit just provides a UI and convenient interception setup tools over the top.