

Judging by the prices in the various countries I’ve lived in, in Europe, mobile data prices are a pretty good indication of a cartel.
In my experience Germany is one of the worst (by comparison to what you quoted, I use to get unlimited 4G in the UK for £10/month some years ago) though my own country, Portugal, is even worse.
I bet there were “radio spectrum” or “mobile operator license” auctions won by a handful well connected large companies and there’s nothing in the law forcing them to open their networks…
Legitimate Interest is an attempt at working around the GDPR using a loophole in the ruling meant to permit processing of data in situations such as when a business has a trading relationship with a client.
However the legal clarification from the EU Commission says: “Your company/organisation must also check that by pursuing its legitimate interests the rights and freedoms of those individuals are not seriously impacted, otherwise your company/organisation cannot rely on grounds of legitimate interest as a justification for processing the data and another legal ground must be found.” (see here) and there is a “right to privacy” in EU law.
So supposedly that nearly endless list of “partners” (read: advert providers, trackers and other assorted businesses who make money from breaking people’s privacy) cannot use legitimate interest to track you as that would break your right to privacy.
That said, in practice they probably do, and until they get fined hard they’ll keep on doing it, so as others said, don’t used a Chrome-based browser and use a good Ad Blocker add-on.