The default alert profile is applied automatically to all sensors in the network, not the organization.
You're correct about the updateNetworkSensorAlertsProfile endpoint, It requires a profile ID, which the default profile does not have since it's applied automatically to all sensors. I apologize for the confusion.
The updateNetworkSensorAlertsProfile endpoint is used to update custom alert profiles, not the default profile.
For the default profile, you can only toggle the alerts on/off and configure recipients. These settings can be accessed and modified via the Meraki Dashboard, under Sensors > Overview > Alert Profiles, by selecting Edit on the Sensor status alerts profile. Unfortunately, these settings cannot be accessed or modified via the API.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.