Is this serious? You would expect them to allow you to customize the domain as they advertise. You can change it but not for the uploaded assets.
Then, they tell you here you can change the provider if you are not happy, but then you have to pay a second service, add more complexity, and they tell you:
The Strapi Support team will not be able to provide assistance to you on any issues that may come about during or after you use an external upload provider
Not happy