I’d like to preface this with my appreciation for Discourse’s latest redesign. I’m digging it. 
As the title suggests, I think some additional tweaking is still needed in the #site-feedback category due to it having subcategories now.
To start, there shouldn’t be uncategorized posts in this section. There’s #site-feedback:misc for that purpose, basically, so every post that belongs in this category should have a proper subcategory already.
There should be a way to disable uncategorized posts on a per-category basis.
Moving on, the whole thing needs some sprucing up so it’s actually obvious subcategories exist. I wouldn’t bother with custom CSS or .js for now, but instead just start with showing the subcategory list above topics, in boxes.
and then, ya’know, edit the pinned system messages to be actually relevant. 
Change the one in #site-feedback:forum to resemble the one at https://forum.smartlydressedgames.com/t/about-the-site-feedback-category/2.