Brilliant work, looking forward to seeing more!

A couple of things. I noticed a bug with marines in the vehicle, which isn't present in vanilla, specifically in the Not A Natural Formation Cave, just before the open area with the bridge. The marines were teleported out of my vehicle proceeding on foot (like a player might be at that loadzone in co-op) but they were still registered as in my vehicle. I still had their health bars, but there was no-one else visible in the vehicle. I only noticed the issue when my gun/passenger weren't firing!
Funnily enough, telling the invisible marines to get out...they do, and become real marines. So, the marine reproductive cycle is confirmed to involve mitosis and a warthog (as expected).

You did a lot of cool things with the blast doors, but it felt a bit immersion breaking that the same thing kept happening. I arrive just as the covenant are beginning to open this door?
If you felt like it, just having crouching marines every now and then with the hacking animation from Lament for Private Jenkins could give some lovely immersion for why the doors open *now*.

With the warthog run, I felt conflicted between "should I be wiping out the covies, or racing to the hangar?". If you want to push the feeling one way or the other, the Halo 2 voiceline of "covie fleet has launched phantoms, too many to track...we don't have much time" can justify a timer like the OG, to get the supplies you need and GTFO. Alternatively, if you want more of a sweep-and-clear feel, maybe some smaller timers. Like, a request for reinforcements from marines at external access junction 4C?