We are sitting at just barely under 100K, so hopefully my opinions will still apply.
We have the full arcgis enterprise, servers, portal, etc. We do not tie any of our arcgis servers or portal into our CAD/911/Dispatch, specifically for the reasons you've stated. We update our addresses and streets into the CAD system on a monthly basis, ship data to the state for NG911, and provide separate shapefiles/gdb for other Emergency Services mapping applications. That way each of those is self-reliant/independent of any other service going down, or it is on the vendor in the case of cloud applications.
Our ArcGIS servers are taken down every morning for data version compression and updates, and then once a month for regular maintenance and updates. That kind of downtime wouldn't work in a 24/7 dispatch environment, and I certainly wouldn't want to be getting the calls dealing with that kind of stuff when outages come up.
The closest thing that we have to what you are describing, is that we have setup a feature service that is for one of our Emergency Services application. However, that service is not directly tied into the application. The Vendor takes our service, scrapes the data, and uploads/hosts the map data on their end. That way it is all self-contained, and we don't have to manually push data updates.
Hope this helps
