I need to ‘wp_dequeue_script’ and ‘styles’ and ADD a bunch of other css and js

Using wp_enqueue_scripts

You should use a proper hook to enqueue/dequeue your scripts, to ensure that it works flawlessly. A proper plugin/theme uses wp_enqueue_scripts to do this, so you should do the same. Wrap your code inside a function, and bind it to this action hook. Also, remember to set the priority to something that makes sure it will override the others.

// Setting the priority to 999 to make sure it runs
// after every other script is enqueued
add_action('wp_enqueue_scripts', 'enqueue_my_scripts', 999);
enqueue_my_scripts () {
    if ( is_page_template( 'specific-template.php' ) ) {
        // Your code here
    }
}