Wordpress - Force Wordpress to Show Pages Instead of Category

One possible solution is to not change the category base and leave as-is, but instead modify any output of a category link to strip the category base via a filter. This of course will result in a 404 if you haven't created a page in place of where these links point, so creating a page for every category is required.

function wpa_alter_cat_links( $termlink, $term, $taxonomy ){
    if( 'category' != $taxonomy ) return $termlink;

    return str_replace( '/category', '', $termlink );
}
add_filter( 'term_link', 'wpa_alter_cat_links', 10, 3 );

You'll probably want to test this thoroughly for any side-effects, use at your own risk!

EDIT - altering just top level category links:

function wpa_alter_cat_links( $termlink, $term, $taxonomy ){
    if( 'category' == $taxonomy && 0 == $term->parent ){
        return str_replace( '/category', '', $termlink );
    }
    return $termlink;
}
add_filter( 'term_link', 'wpa_alter_cat_links', 10, 3 );

I created the following, fairly simple function in order to make the pages a higher importance than categories when loading the content.

(Note: this may not the best, but a simple way. I found an issue, when using the Advanced Custom Fields plugin, it doesn't insert the custom fields neither of the old, nor of the new page by default. Adding get_queried_object()->ID as the ACF get_field()'s second parameter solved this. Test it carefully in your WP environment.)

function loadPageFirst() {
    // get the actual category
    $actualCategory = get_category( get_query_var('cat') );
    // get the page with the same slug
    $matchingPage = get_page_by_path( $actualCategory->slug );

    // If no match, load the normal listing template and exit (edit if you are using a custom listing template, eg. category.php)
    if (!$matchingPage) {
        include( get_template_directory() . '/archive.php');
        die();
    }

    // Make a new query with the page's ID and load the page template
    query_posts( 'page_id=' . $matchingPage->ID );
    include( get_template_directory() . '/page.php');
    die();
}
add_filter( 'category_template', 'loadPageFirst' );

You can include it in your theme's functions.php. This looks to work OK with the Yoast SEO plugin.