Auth Connect doesn’t require the Ionic CLI, you can use it inside of a Capacitor application that isn’t using Ionic at all. It’s probably your best bet since it supports several SSO options.
However, if you don’t have Ionic Enterprise, there are a ton of different SSO providers so it really depends on what provider you go with. Auth0 has a quickstart guide for Capacitor & React if you use them as your SSO provider and I’m sure other providers have similar tutorials/libraries.