Given how little we know about the Cabal, this post really ties together the information we do have nicely! The information about natural resources was also very illuminating, as well as being something that doesn't often get discussed. However, I do think that the Cabal's discovery of the Risen happened before the Guardian broke through the Exclusion Zone. From Ghost Fragment: Titan-
/ Cabal move slowly. We've got time.
/ But the Warlocks have had a vision.
/ Yes. That new one, what's her name. Ingora?
/ She's always been hasty. I've faced these Cabal before. I know 'em like I know my own armor.
Holborn's Host has been active for longer than Ikora has been Vanguard, meaning that (if Ikora was Osiris' immediate successor) the Hosts have been on Mars since at least the City Age. The existence of Blind Watch also implies that there was a Guardian presence on Mars before the vanilla campaign:
The Blind Watch was once a clandestine Guardian outpost on the outskirts of Freehold, used to monitor Cabal activity in and around the Buried City. After the Cabal consolidated their forces nearer their fortified Exclusion Zone, the site was reclaimed by the Crucible as a combat training ground.
Overall, I do think that the Cabal came to our solar system for resources, and ended up in the middle of the Vex and humanity. However, the Cabal's attrition war has presumably gone on for much longer than the events of Destiny One, which certainly leaves room for more speculation as to why the Cabal didn't send in the calvary sooner. All in all, this was a great post!