I’m not sure that Discourse Templates would help here as it requires a user to actively select a pre-existing template whereas I understand what is needed here is to provide immediate feedback to the user once a category is chosen for their new topic.
To conditionally replace the text that shows up on the right, I think it’d require implementing a theme-component or similar.
One option would be to use the built in “Topic Templates” feature for the category, to pre-fill the composer with some text to fill in or replace: What are Topic Templates?