Disable node summary using Page Manager There are a few modules and methods specifically for disabling / removing the node summary page (at /node), like Node Page Disable, but I'm wondering if there's a way to redirect or override the page using the Page Manager module.

There's a good summary of other methods on this stack exchange question and answer and this Drupal forum post. One problem I'm running into with Page Manager is that the /node/%node path is already in use, and I'm not sure how to set it up for just the /node page without any additional arguments.
Yes, you can do that with a variant. I've used that methodology on a personal project. Here are screenshots of the configuration.

The first variant (which returns a 404) checks, the node type (I have a content type called 'person'), the taxonomy terms applied to the node, and the role of the user.