Thank you for the response! A cheaper alternative or going open source is something we are considering and this is what we might end up doing.
Before we finalize that decision, I am required to fully workout and expand on this option to my team. Suppose we use an online service such as Okta to handle authentication and user registration and then customize/develop the portal in-house to integrate this into the web pages/forms, it is against Power Platform's terms of service to do this and basically bypass Authenticated Users functionality?