how do I get an alert if our upload bandwidth used crosses a threshold?
You want an edge router to shape the traffic, set quality of service and optionally have alarms. These are extremely reasonably priced for the features they allow.


https://www.amazon.com/Ubiquiti-Networks-ER-X-Router/dp/B0144R449W/


Some of the alerting won't be needed since you can shape the actual traffic rather than go react to it.