you can try this plugin:

https://www.ads-software.com/plugins/category-posts-slider-pro/

Or try this one also
https://www.ads-software.com/plugins/advanced-post-slider/
But it hasn’t been tested with the latest 3 major releases of WordPress, So you can try the first one.

Neither of those plugins seem to offer what I’m looking for.
The first one seems to select posts within a category and make a slider from them, and the second will only let you place the slider on posts, rather than categories.

I’m thinking what I want to do isn’t possible..
I just want a single banner for each category, to display on the posts within that category. As I mentioned, some pages are within multiple categories, making it difficult.

I think I will have to create multiple posts of the same product, and just change the banners on each one :/