I think you may be underestimating Johann here. He's not a himbo, he was a fully qualified Magister trusted with a dispensation to push the boundaries of what's permissible despite his College thinking that he's magically disabled before he even met Mathilde.
I would expect him to have his own agenda and projects that he'll be finding a way to advance. This is an opportunity for Johann, given how unwelcome male arcane magic users are usually made in Kislev. An army may not be the easiest place to do so, but there are a fair number of nobles here to talk to.
As an example, the Gold College is a major commercial operation, producing and selling all sorts of chemical products, from dyes to soap to distilled spirits, to alloys, and even to gunpowder (although with a restricted sales list). Johann would probably make a very good salesman.
There are also personal projects he could be working on. We don't know what they are, I think, as I believe we've not had chance to spend a social action that might tell us.