Show customisable message on private topics/categories?

[Stephen]

You could achieve this with a category banner on the parent categories:

Discourse Category Banners

This theme component uses existing category details to create banners for your category pages. It uses the category name, description, and color to generate a banner at the top of the relevant category pages (for tags check out Discourse Tag Banners). The category description is defined by the first paragraph in each category’s “Category definition for …” topic. The banner background/text colors are defined by your category badge settings. [54%20PM] By default this will display on desktop an…

People outside those groups wouldn’t see the subcategories at all because of the way group ACLs work.

But in each case you would need to customize the banner heavily using CSS and differentiating between users who have access and those who don’t would require further work.