As an update I did find a ESRI blog post the other day talking about publishing Subtype Group layers to enterprise portal it stated the following: It is possible to publish a Subtype Group Layer to Enterprise, however, it must be published as a non-hosted feature service. This means the data needs to come from a registered enterprise geodatabase. I haven't had the time to test it to see if Subtype Group layers would work specifically with publishing the UN. If it works this would give you the ability to maintain the domains and have symbology by asset type or any other attribute you choose.
At the time we weren't aware of the Subtype Group layers being a possible option for services on enterprise. Our current setup is as follows. We have a UN service that is published up by asset group on portal with the basic UN symbology included (in the Solution it is the map with the black and white symbols). This is the service we use for editing in ArcGIS Pro exclusively. I pull those layers into Pro as subtype group layers and symbolize on asset type or other attributes depending on the editors needs. This service also includes the Utility Network layer that manages the dirty areas and network topology as that is fully supported when editing UN in Pro.
We then have a second UN service published to portal same as above by asset group with basic symbols. This is the service used for field collection maps/apps. As you may know the UN is not fully supported in the current Web Map Viewer, Field Maps and other web apps. So this service only has the feature classes of the UN (Devices, Lines, Junctions, Assemblies, Structures, etc.) and not the Utility Network layer. From here I pull these layers into a web map and use Arcade code to classify out the layers by asset type or various other attributes, and also use Arcade code for the labels. You may find you need to pull in a layer multiple times to get the feature classification required. You can also open the web maps in Pro and make all the adjustments to popups, labels, and symbology there. That is often easier than working in the web viewer. Overall it has been a somewhat frustrating process it doesn't help that there are currently some quirks/bugs to Field Maps and the Map Viewer where certain things just don't work as expected in the Portal environment vs AGOL. If you have the resources it maybe best to have multiple services one for each specific web use case, however this obviously uses more resources and is harder to maintain if schema changes occur. I think it is also worth giving the Subtype Group layers a try and see if it works with UN when publishing up.
