This one's a bit out of left field, but would it be possible to lock Aestusium and Frigusium behind technologies? Starting tech feels pretty real-world realistic in most cases, but those two magic gases are available right at game start, and that feels too easy to me. Nothing too crazy, maybe 5k RP each, but a bit of a speed bump. (Plus, it'd fill out the Biology tech tree a bit.)
Also, is there a reason why water vapor isn't a greenhouse gas? If that causes mechanical issues, that's fine, but otherwise I think that might make a good safe greenhouse gas for us to crank out in the early game, to make up for the lack of Aestusium. It's certainly a powerful warming agent IRL. CO2 plus H2O is probably enough for Mars and Luna to get habitable if H2O is turned into a GHG, and it'll at least make a dent in the CCs for the moons of Jupiter and Saturn.
(And while we're playing around with mechanics here, maybe make sulfur dioxide an anti-GHG? Again, it certainly is IRL, and it'll add a few options for terraforming.)
The 'magic' gases were added because there are no good options for real GH and AHG gases. CO2, Methane, SO2, etc. are all dangerous gases, so even if you terraform a planet, it will still be CC2+.
Water Vapour is the only one that isn't, but it is involved in the evaporation and condensation cycle, so it will quickly exit the atmosphere in most cases.
I could make the 'magic' gases researchable, although I would probably reduce the cost of terraforming modules as well to make sure that terraforming isn't too expensive. I understand the point though about them being not real world at the start.
Oh, I totally get why they exist, and I agree with your logic. I'm suggesting a tweak here, not going with RL physics. (If it was RL physics, we'd need to figure out how exactly gigatons of oxygen are just showing up or disappearing on demand - Aurora is not the kind of game that wants to focus on clever soil chemistry like that.) And yeah, reducing costs of related techs to offset this change sounds perfectly reasonable to me.
Using CO2 or SO2 to warm/cool a planet can be useful for dropping the CC from (say) 5 to 2, but it won't get you to 0 the way the magic gases will. But I think that could be an interesting way to start the game, using the inferior gases to get started and then correcting them later on. 5->2 is still a big improvement, after all.
As for non-terraforming options, I could see something like a solar mirror module for station use. Probably like 100k tons, 200-500 cost, and they can either raise or lower planetary albedo by (say) 0.01 immediately. That gives you an option for instant changes to a world, instead of waiting around for the atmosphere to change. It also gives you an option to switch modes for very eccentric bodies (block sunlight when it's close, reflect more sunlight down when it's far away). The drawback is that it needs to stay there permanently, instead of being able to fix it and move on like a terraforming ship/installation can. (So you'd probably use that to get started, then terraform under the mirrors until you're in a stable place, and *then* move the mirrors away). Also, this multiplies with GHG effect, so you can use it to get more warming/cooling than the x3 from GHGs would allow - that'd be useful for some outer- system planets that are too cold to ever warm up with current mechanics.
As for the vulcanism thing, that could maybe work? Probably a lot of work to make it viable, though - you'd want some kind of flag for "tectonically active" in the body generation mechanics, which would need to be displayed somewhere. And that'd just get you a burst of toxic cooling gases, which are generally a lot less useful than warming gases.
If you did want dust as a bigger mechanic for players to terraform with, though, there's another way - mass drivers. Let mass drivers fling inert rock at bodies freely (without needing to mine the rock), so you don't need to waste perfectly good minerals on it, and have that kick up a lot of dust.
