I find this very interesting. Disclosure:I'm basically English monolingual, but my English is a melange of different kinds, American, BBC, high and low brow, and littered with pilfered vocabulary. The normal prescription for writing is to read a lot, and that this builds a buffer to push your writing through. However. English has a Huge body to select from, deep and wide. I don't know how one improves writing in a language with (from your description) few suitable models. I suppose you might try 'exercises' where you set a desired effect and try different approaches.

Perhaps trying to write the Danish you use when the audience doesn't know English?