Moving Quantity after product description

Easiest way to make it done is by playing with hook priorities.

On content-single-product.php is this comment:

    /**
     * Hook: woocommerce_single_product_summary.
     *
     * @hooked woocommerce_template_single_title - 5
     * @hooked woocommerce_template_single_rating - 10
     * @hooked woocommerce_template_single_price - 10
     * @hooked woocommerce_template_single_excerpt - 20
     * @hooked woocommerce_template_single_add_to_cart - 30
     * @hooked woocommerce_template_single_meta - 40
     * @hooked woocommerce_template_single_sharing - 50
     * @hooked WC_Structured_Data::generate_product_data() - 60
     */

So to put something after excerpt, it needs to have the priority between 20 - 30. First of all you need to remove woocommerce_template_single_add_to_cart which shows the quantity and add to cart button.

remove_action('woocommerce_single_product_summary','woocommerce_template_single_add_to_cart', 30);

Then register this same action but with higher priority which shows the quantity and add to cart button after excerpt.

add_action('woocommerce_single_product_summary',woocommerce_template_single_add_to_cart', 20);

Since you want to move just the quantity, my solution would be to register the action again .. the priority is up to you.

add_action('woocommerce_single_product_summary','woocommerce_template_single_add_to_cart', 30);

and with simple css hide the button after the excerpt and below hide the quantity.