I don't know if there is an omnibus or not yet but when I first got Deathwatch, I bought and red the Space Marine short story anthologies: Heroes of the Space Marines, Legends of the Space Marines, and Victories of the Space Marines. Each story is dedicated to a different chapter. Some of the stories sucked but others were really good and gave me a good taste for the different marine chapters.
The video games are good for space marine fluff too since they all kind of focus on the marines: Space Marine, both Dawn of Wars (and all the expansions) and Kill-Team are all pretty decent and give a good taste of space marines and lots of Xenos fighting. You mentioned using the Tyranids, the vanilla campaign for Dawn of War II is all about the space marines fighting the Tyranids so that might be a really good game to play if you're looking to get some extra flavor.
I'm reading the Word Bearers omnibus right now and it has been giving me all kinds of good ideas for using Chaos. As far as the books go, there is a lot of stuff out there and they have done a good job covering different topics (trilogies related to certain factions, marine chapters, etc.) so there's a good chance you can find a book or three covering the same kinds of stories you want to tell.
The campaign books that FFG has published are all pretty solid too. I haven't picked up the latest one yet but I've liked the other adventures. The Jericho Reach includes an adventure that deals with the Tyranids, so it might be worth checking out (the rest of the book is just extra fluff on the setting which is of so-so use)