I don't see any RNG in what OP says.
Beyond that, i dont believe a typical legendary h2 run has much of any rng. A few things here and there but thats it.
Speed running it, ehh maybe a bit more technically.
Enemy ranks and spawning, trying to stealth the prison area in The Gravemind can be instantly screwed (with no other choice than restarting the mission) by a Jackal having the gravity lift in sight, sometimes nobody is watching. I like to have a good chance at going to a lower level and take everyone at the middle.
In Uprising, one Grunt spawn is always coming with a Fuel Rod Gun, not the others. That's +20 shots, I don't say no even if it's doable without.
Marines in Delta Halo can actually spawn with a Rocket Launcher (with the Scorpion drop, I can't tell for the beginning) , Extra firepower but not needed with the tank.
The player's life can end fast if an enemy Shotgun decided to have good accuracy, it can last longer when it's a sniper who is actually missing a shot. Don't test your luck.
Covenant vehicles are easier or harder to destroy (the rank of the Elite inside) . When I'm about to reach Regret's temple, I want Banshees that go down in one or two rockets, the 2 FRGs are trigger happy once they start firing and the 2 Rangers are very likely to come when I am hiding.
Enemy sniper spawning at the beginning of Outskirts and Regret, in Outskirts it's 100% random (when and where) , in Regret only the locations are random.
I had a Flood with a Sniper Rifle in High Charity, in the first room after the one with the crystals. It was visible unlike that guy much later but it was a very unique spawn to me.
The small things add pretty fast, I can add more and yeah Halo 2 MCC is lacking dual wielding Elites otherwise they never overheat with a Plasma Rifle on the left hand.