If you own your own domain, it's trivial for them to notice that it's a vanity domain for a single user or small group. The only way to hide is to have thousands of friends with diverse interests using your domain.

Trivial to a human looking at the data set, but it's something the algorithm would have to be trained to recognize.

Not really - in practice the tracking system can have a list of email providers and treat every other domain as one user (especially ones with catch-all setups).

I don't think it's trivial at all? I sign up with random first names and last names, my email address doesn't have the actual company name and I don't think there's that many people with vanity domains to justify adding code to track this use case.