Send email to all users with specific role (updated privacy policy) We want to send an email to all users of a given role, to let them know about an updated privacy policy.

The filter by role allows to send different versions of the email to different roles.. not sure this is needed, but it will be useful to know.

Since this is a legal notice, it does not make sense to provide an unsubscribe link.

The email should be a somewhat decent looking HTML.
Perhaps we want to add some user-specific tokens, e.t. to link to the user profile.

So there needs to be some kind of authoring UI, e.g. by creating a node or something.

What I looked at so far

I installed mass_contact, but it seemed to add unnecessary complication with its categories.

In some other questions, people suggest to use rules. But in my understanding those are mostly meant for repeated email-sending, not for a one-off. Also I don't really know where I would find the authoring UI.

Simplenews might be another option. In my understanding, this is designed for consent-based newsletters.

Perhaps all these solutions can be used for the purpose I intend, but perhaps there is something more specialized for this specific task?
Mass contact is probably the way to go for you, if it has to be a solution integrated into the site. Categories are really easy to configure. 

Automated and integrated solutions are always recommended over hacks, but if you're really doing this only once, the fastest way would be to export email addresses and use an email campaigning service or design and send manually in batches (sending bulk emails are limited per hour for spam reasons) from your site email address but that is difficult to test. 

Setting up the HTML styling for your email is what you'll spend most of your time (through Mime mail module). If you plan on testing it on various email clients that is. Many services (Gmail for example) strip styling from emails, so you have to be careful what you style and how. Usually you use a tool that takes your email CSS and applies it all inline, because that is the most reliable way most of the styles will "survive" various email client filtering. 

Simplenews is a very comprehensive solution, much much more to configure and understand there than Mass contact. 

I can't see configuring Rules to be triggered would be easier than using Mass contact. 

In the future you may consider using a module like Legal.