The question you have to ask yourself if you go this direction is, "Is Supabase better than Amplify?"

Supabase uses a relational database as its backend which is convenient and quick when working with relational data, but will it scale to terrabyte sizes? Probably not affordably, and not without actually having to hire a DB admin and do all of the dreaded 'backend development' that you hoped to avoid. Amplify uses DynamoDB out of the box which is setup to cheaply scale to terrabyte sizes. The downside being, it's a little trickier working with relational data.

Amplify also provides all of the same bells and whistles mentioned. Realtime pub-sub out of the box. An Auth service. Support for creating Lambda functions easily. Not mentioned is Amplify supplies easy access to S3 and SDK components to make secure and CDN backed document storage possible.

The biggest upside I see with Amplify is that it is integrated in the AWS ecosystem. When building Lambda functions you can take advantage of all of the offerings AWS has on the table including Machine Learning, Push Notifications, Geolocation functionality and more.