How to Implement Scalable Custom Scheduled Push Notifications in Flutter Using FCM Without WorkManager?

Question

I’m developing a Flutter app where users can set custom schedules for receiving push notifications. Each user can choose their own frequency, such as:

10 notifications per day

5 notifications per day

1 notification per day

3 notifications per week

I want to use Firebase Cloud Messaging (FCM) to send these notifications, but I want to avoid using WorkManager or similar background task solutions due to battery restrictions and Doze mode, which can cause the app to stop working properly.

I’ve considered using local scheduling with flutter_local_notifications, but this approach has limitations: Notifications may not trigger reliably if the app has been shut down or the device is in Doze mode(battery restriction).

Is it possible to use Fcm(or anything like that), especially when the number of users increases?

Is using Firebase Cloud Functions a cost-effective solution for this use case? If not, what are some alternative approaches that balance scalability and cost?