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.