After some digging I found some additional information which I want to add to the thread:
A ticket at notify_push where someone requests something similar, integrating some statistics into NC to get the information easier
Quite a simple way to see some metrics, and see if it works is by using occ
sudo -u www-data php /var/www/nextcloud/occ notify_push:metrics
Active connection count: 4
Total connection count: 47
Total database query count: 129
Events received: 217602
Messages send: 4417
Additionally as mentioned above the debug option for notify_push
sudo -u www-data php /var/www/nextcloud/occ notify_push:log debug
sudo journalctl -eu notify_push
I hope this helps someone 