You can always create a topic in a public category and link to that from various places like the first sidebar section. But then it would look like a topic in the forum and not a standalone page. That might be acceptable to you.
There is currently no way to create such a topic and add it to the navigation menu so it appears as a page alongside the about page, terms of service and privacy notice. I think this would be an interesting feature request. Then you could have this:
One solution that comes to mind is to switch to the experimental about page, which soon will become the only about page, and then to edit site texts to edit the “Contact us” section on the right side so it includes full impressum details. To do that, look for “site texts” on the sidebar, switch to your site’s language, then search for “if you come across”. You can also rename the “Contact us” header to “Impressum” by searching for that.
If you need more space, you could also add impressum details to the site description, which appears directly on the login page on private sites and on the about page.