Plugin to reformat the post automatically after submitting

You should use the transition_post_status to filter the content after submitting the post.

Try the below code in the functions.php in your theme:

add_action( 'transition_post_status', '_new_post', 10, 3 );

function _new_post( $new_status, $old_status, $post ) {
    if ( 'publish' !== $new_status or 'publish' === $old_status ) {
        return;
    }

    if ( 'post' !== $post->post_type ) {
        return;
    } // restrict the filter to a specific post type

    $plain_content = strip_tags( $post->post_content );
    $new_post      = array(
        'ID'           => $post->ID,
        'post_content' => $plain_content,
    );

    wp_update_post( $new_post );
}