Can I designate a filtereable portfolio to show only a subset of porfolio items (category is ‘featured’) & pick which categories to filter by (there’s 4 others I want to filter by, but I don’t want ‘featured’ in the list)
You can use the [show_post_snippets] to control the items displayed by category by using the parameters taxonomy and terms. The list shown will not be filterable though. Pls have a look at the documentation for more details on these two params for the [show_post_snippets] shortcode.
You may also consider using Essential Grid plugin for your needs. It lets you select the categories to be shown and also lets you filter the items. It is available on CodeCanyon for purchase.