This game is a little like games like dwarf fortress where you are playing to inevitable death, it just depends on how long you last and how great you become before the imminent downfall.
Just a couple tips I've picked up:
1- You lose when your Chapter Master dies, so if you want to prolong the inevitable, don't take him into battle. He's really buff and can help a lot, but an unlucky space battle can end the game over a single lost ship.
2- Don't try to solve all of your sector's problems at once. You're much more powerful patrolling as a full (or nearly full) chapter than as a bunch of mismatched companies, and for big ork infestations and big tyranid infestations you'll want the full chapter.
3- Avoid space battles as much as possible. Orks crash their ships into planets which drops their fleets to zero, so when facing orks, its much better to just let them arrive, crash, and then either attack or bombard the planet.
I've got a question for everyone: has anyone ever encountered Eldar? I've faced Orks, Tau, Chaos, and Tyranid, but never Eldar even though they're in the diplomacy menu. Just wondering if that's an existing feature in the current game that was removed thanks to one of the patch things I (like how adding the "fix graphics" patch caused me to no longer start with any relics).
Lastly, the rewrite is coming along. Nothing really to show off yet since all the work is in the code end and not the UI end yet, but it's coming.