> My target audience is the data consumers, and the language to tell them is the OSM tags.

And when you are using the wrong tags, like you do now, they can't process them properly. The renderer is one such data consumer, just telling you. 

There is a taggin scheme, tunnel=* but simply chose to ignore it because you chose to use the description and ignore the whole text there. The funny thing is - if you do the same with the wrong tag you used, its pretty clear you can't use it. https://wiki.openstreetmap.org/wiki/Tag:waterway%3Ddrain "An artificial free flow waterway used for carrying superfluous water, usually lined with concrete." - "Frei fließend". 

And if you want something additional you have to do what you suggested others - make a proposal for the new tagging scheme for sewers. Maybe even participate in https://wiki.openstreetmap.org/wiki/Proposal:Culvert_type for example. 

It seems like this should be tagged as culvert. 