I have an azure subscription that has upwards of 200 appServices where around about half of them have Continuous, always on webJobs attached, some also have slots which also
You can do it through the ARM APIs, though you still need to call it on each Web App.
You can get the WebJobs with a GET request to:
https://management.azure.com/subscriptions/subscription-id/resourceGroups/resource-group-name/providers/Microsoft.Web/sites/app-name/webjobs?api-version=2016-03-01
But I doubt this will be any more efficient than what you have since you still need to make a call for every Web App. And you will need to get the access token somehow.
Web Jobs are a property of App Service applications and can't be requested all at once from Azure.