@MihaelIsaev it appears that the approach you took for sending messages to multiple devices is to create a topic for each group of 20 recipients, then for each topic send the message and delete the topic afterwards. Although it works, it seems a bit fragile and suboptimal. Firebase accepts sending messages to up to 500 recipients at a time using a "mixed/multipart" request to https://fcm.googleapis.com/batch:
https://firebase.google.com/docs/cloud-messaging/send-message#send-messages-to-multiple-devices

Have you experimented with that?