Can you provide an example of the website which makes it impossible to insert JS from extension? I'd like to tinker with it. I wrote a simple extension which inserted JS to youtube and it worked just fine. I thought youtube is secure enough?
For a Chrome extension using manifest v3: you can inject script tags with a src pointing to a local JS file in the extension regardless of the CSP.
Modifying the CSP is not a requirement for a Chrome extension to interact with a page's JS. I don't know about other browsers though.
In order to inject my code before theirs, I had to come up with different approaches on FF and Chrome... and since it was starting to become a cat and mouse game, I just took the extension private.
After that, they didn't escalate their game. I'm thankful for that, so that I can keep enjoying it without spending lots of time figuring out how to mess with it.