Combining Firebase and .NET Authentication for Push Notifications

So I have a group of users who have been using a .NET application forever via simple username/password authentication. I’m sketching out what it would take to bring the experience into an Ionic App and need some guidance on should I use Firebase for authentication just for ease of sending push notifications?

I can already create bearer tokens on the .NET API side. I can also validate the Firebase Auth Token from the .NET API as well. I would prefer not to use Firebase since it’s an added cost, but for push notifications I can’t seem to find anything for ionic since the dismemberment of the ionic cloud service.

Ideally, I’d love to keep my bearer token authentication so everything is authenticated on the API side and simply find a simpler, better way to send push notifications. Any suggestions?