Hi,
The HTML of your add to cart buttons is the HTML from the legacy add to cart system.
So if you still have that HTML even though you have the add_to_cart_legacy entry to 0, it means that you have view overrides preventing you from using the new add to cart buttons.
You'll have to check your view overrides.
The problem should come from one of the files in templates/YOUR_TEMPLATE/html/com_hikashop/product/
So try renaming them one by one until you identify the one which will give you the new add to cart buttons.
Once you identify it, you'll probably want to get the customization you had in it and add it to the new version of the view file via the menu Display>Views.
That way, you'll get the new add to cart button, which will take into account the settings of the cart notification plugin, and you'll get your customization.
From what I can see on your website, it's probably the quantity.php view file.
I see that you're hiding the quantity input field area with CSS and you've added some custom CSS on the add to cart button. For the quantity input field, no need to HTML/CSS customization. You can just turn off the "quantity input method" in the HikaShop configuration and it will remove the quantity input field on the product pages. So you'll just have to modify a bit the CSS to customize the new add to cart button color, border and width and that way you should be able to get the same result quite easily.