Processing or possibly even keeping a birthdate for an astrology newsletter is clearly a legitiamate interest for the subscriber of that newsletter.

> but it’s a valid business model.

What is? Showing ads to provide a service is a valid business model yes. Showing tracking ads or blocking those who don't accept the ads - no.

But "I need to show the ads to keep the lights on" is NOT a legitimate interest to the visitor. The reason for handling the personal infrmation needs to be a hard requirement to provide service itself. Not merely part of the "business model". You cannot set up a separate service (paid subscription) and argue that because that other service exists, your ad-funded service deserves special exceptions from the GDPR e.g. that it can show ads which are tracked or else users are blocked. It's pretty clear in the regulation that "cookie walls" aren't allowed, just like pre-checked/assumed consent isn't.