There are other benefits to using the Web Adaptor internally, as well. For one thing, it can be used to host the ArcGIS Services on port 80 instead of 6080, helping mask the actual server endpoints. This also lets you restrict access to the administrative REST interface to port 6080. Most importantly, you can use it to set up Windows Authentication in IIS. In general, the Web Adaptor acts as a proxy to your actual ArcGIS Server. I think you still have some more architectural options externally, but I don't know architecture well enough to make any recommendations. Definitely, consider the Web Adaptor internally, as well.