In ArcGIS Enterprise 10.6.1 it's near-impossible to publish a non-hosted WFS service - the only way we managed to accomplish this is by publishing through ArcMap, which, as a Pro using organisation, is rather nasty.

The difference between a "Pro" and a "map" published service is night and day, they have a completely different capability selection in Server Manager:

vs how it looks like through ArcMap:

Both are further on equally registered to the server. It's almost as if the UI doesn't have room for more than eight capabilities.