I think your motorway driving is sufficient to keep the DPF in good order - particularly if you occasionally "let it have a deep breath"*.
* These were the exact instructions for good engine life in an early BMW 3-series. As you can see, BMW weren't too hot on translation back in the 1970s. It was printed in the handbook in wonderful bold gothic script. I think the full sentence was something like: "At least once every month, guide the car to the open road and let it have a deep breath". Words I've lived by ever since.