Set homepage to only display posts from one tag

You should use pre_get_posts to alter the main query on the home page.

With the proper conditional tags and parameters (check WP_Query for available parameters) you can achieve what you need

You can do the following to just display posts from a given tag on your homepage

add_action( 'pre_get_posts', function ( $query ) {
    if ( !is_admin() && $query->is_home() && $query->is_main_query() ) {
        $query->set( 'tag', 'SLUG_OF_TAG' );
    }
});