Wordpress - How would I get a taxonomy/category list inside a Gutenberg block?

You don't need apiFetch or localization to get a list of all categories. You can do this with the wp.data module:

wp.data.select('core').getEntityRecords('taxonomy', 'category');

See the Gutenberg Handbook entry on Core Data for more details.

Load the elements into a constant using a function like this:

const postSelections = [];

const allPosts = wp.apiFetch({path: "/wp/v2/posts"}).then(posts => {
    postSelections.push({label: "Select a Post", value: 0});
    $.each( posts, function( key, val ) {
        postSelections.push({label: val.title.rendered, value: val.id});
    });
    return postSelections;
});

Then use postSelections as your element "options".

                    el(
                        wp.components.SelectControl,
                        {
                            label: __('Select a Post'),
                            help: 'Select a post to display as a banner.',
                            options: postSelections,
                            value: selectedPost,
                            onChange: onChangePost
                        }
                    ),