Just to give general ideas of where you can go, MiniWarGaming has done a two campaigns from a Deathwatch perspective. The use a lot of custom rules, and have a major branching story of mystery and plot twists, but it is a great watch and could give some inspiration there.
Here are their videos:
https://www.miniwargaming.com/show/augustines-station
https://www.miniwargaming.com/show/space-hulk-deathwatch-narrative-campaign
The Space Hulk one is a sequel to Augustine's Station.
The question I have: are the Marines on the same side? With the Dark Angels, (and with Chaos waiting in the wing) you could do some shenanigans with the Fallen that sends a simple mission to wipe out Orks on a planet spiraling out of control as the would be Heroes are at each other's throats. This is also a great opportunity to use the rivalry of the Dark Angels and Space Wolves to your advantage.