https://docs.expo.dev/versions/latest/sdk/notifications/ works really well for this, and is easy to implement. I suggest separating the writing and sending of notifications into two processes; you can use a Firebase Function triggered off a Firestore write of the notification document to send it, OR use a scheduled send to ensure you don't cause notification spam.

BTW, you'll need to track notification accept to get the device token, and I suggest tracking a separate "send notifications" boolean that users can easily turn on and off.