Hey, I like the sound of your scenario. It seems like you are concentrating very heavily on the Space Marines and not very much on the Orks. I would recommend, especially if you are using these as introductions for new players, to also put some thought into what the orks are doing to overcome the obstacle in their path should they fail to break through. Because sometimes players want to play the "bad guys" and that's a valid option in
40k.
Do the orks add more boyz to their Assault? Do their break-throughs have any impact on an ongoing narrative? Maybe if you start with this scenario one week you could play out the next stage of this battle using orks that broke through last week versus a new type of gun line?
I will always advocate for more narrative over game-ification for building enthusiasm in new players.
Very often terrain can have an affect on game play. Does that valley mouth favor the defenders (it should, they're smart space marines)? And can the Orks have special rules that let them try to damage or destroy the cover that the space Marines are using? Just some ideas.
I wrote dozens of custom scenarios in the
Campaign I posted on this very forum. Go check it out and steal all the ideas you like. Keep at it and have fun!
Automatically Appended Next Post: Also, I'd love to know the names of the planet, the mountain pass the Marines are defending, maybe even the names of the marine defenders themselves. What's the backstory? Who asked for help defending their world and why did this Chapter respond? Do they have ancient Oaths to uphold? Are they here for a more sinister purpose? Is it just chance that they happened to be near enough to respond to the Astropathic call for help? Did they arrive in time to save the populace or is this a war for vengeance?