I'm going to offer a full website proxy service for a small price so the external JS isn't needed and hopefully these cases can be mitigated.

But surely it is dubious (another reason to fear and dislike the GDPR as it is) if a visitor is actively bypassing website contents and features - effectively changing the website - and that makes the website liable.

It's like if I have a browser extension that rewrites landing page contents to racial offenses, then I browse landing pages, get offended by them all and sue the websites.