When you’re in your Woo layout, there’s an element called “Add to Cart Form”, and this pulls in most of the advanced customizations like that.

You basically use this rather than setting up one of Pro’s custom buttons for more complex products like you’re describing. I haven’t gone too deep on this yet, but I think you might be able to use the new conditions system to show this form only when there are bundled options.
This form uses the styling inherited from whatever Stack you have selected in the Theme Options, so you’ll need to mess with those settings or write custom CSS to change the styling of the buttons/forms.