List cloud notifications services with push, SMS, email, slack etc.

I’ve used OneSignal and their generous free tier in the past with great success. It has nifty features like using an external user ID (your backend can just say: send a message to @user123). I had to switch to Firebase Cloud Messaging for compatibility reasons but I still highly recommend OneSignal.

Since you're looking for free or low cost, how about self hosting?

Gotify allows push notifications to Android.

NodeRED can expose an API and trigger actions such as SMS through plugins.

You could try Twilio. Pricing depends on the volume of notifications you send. It supports both apns and gcm. The documentation is good.

Ifttt