Smush Pro is a great plugin. When someone buys the license to use this is because they know how important it is to optimize your site. With such a mindset, using a CDN service is just one step.
So if I believe optimizing and offering my images is very important, I’ll use a CDN as well. And, if I have a contract with a technology company to serve my application, why should I put my media files in another company?
I want to have more bargaining power with my suppliers, so I focus services in one place. However, with this scenario Smush Pro loses its relevance because it prevents me from doing a good business. And getting a new third party plugin to do the integration with my CDN, besides looking like a denescessary expense is also a bump of code for my application.
So my suggestion is that you do the native integration with the most used storage services: S3, Spaces and Google Storage.