Creating your own custom portal always means a huge investment and an ongoing process of supporting each update being installed on Azure Stack itself. Each service you would like to offer to your customers with the custom solution needs to be developed. This means that the developers need to understand the API, the way Azure Stack works, and how to code against this solution. From real-world project experiences, I know a custom portal is possible using the APIs, but the question should be more like, whether it is worth it, taking into account the amount of money that needs to be spent in the form of development hours and manpower.