I’d caution anyone against thinking they can save a few $$'s by using one authentication system now then replacing it with another later. Authentication systems tend to be central to any product. Trying to retro fit or replace your authentication is probably the single most expensive, time consuming and disruptive thing you will ever do.
As @jonsmirl pointed out Amplify and Cognito have improved a lot in recent months but they still suffer from a limited number of providers. His advice to start off with Amplify unless you need a provider they don’t support is spot on.