Hot take: Iron Gold is better than Dark Age ... like a lot better.
This is just my opinion. But I recently finished Dark Age -- and this might be a hot take -- Iron Gold is better.
Maybe it's a function of where my expectations were set for each book. Many people seem to dislike IG, and rave over DA and LB. So it's not super surprising that IG was a pleasant surprise and DA was a disappointment. But even controlling for that, I think IG is just a superior book.
IG has better story-telling. Each main POV has a unique journey, each with multiple sub-plots. They each have a very unique voice. The ways the stories connect is surprising. The climax for each POV is unique to the character and yet also quite satisfying. The story is just so much more dynamic than DA. It also sets up some great intrigue for the next books. I had a great time!
By comparison, DA felt stagnant. The adventures are darker and more violent, but they are less dynamic. Darrow and Lysander are confined to Mercury, and spend nearly all of it fighting or recovering from fighting. (By comparison, Darrow's story in IG starts as a political thriller, leading to a jail-break, and culminating in a major battle on Venus.) Ephraim's story feels like it's more about introducing Pax than anything else, a big step down from his story in IG (a heist plot and a story about PTSD and self-forgiveness). And although I really liked having Virginia's POV, I just don't think it makes up the lost ground. The characters also don't really do as much by comparison (the exception being Lyria, who I think got a big upgrade in DA in terms of having things to do within the plot).
I also think the supporting cast is just more interesting in IG. Appollonius, the Duke of Hands, Cassius, the Au Raas, Kavax. All of them were fairly electric on the page. I'm not saying the supporting cast is bad in DA--I liked having more time with Sefi and Victra--but I just wasn't as interested in the supporting characters in DA as compared to IG. It kind of felt like many of them were introduced to the story to serve as fodder (RIP the Skuggi). And the combat on Mercury really flattened the characters around Darrow and Lysander.
I also just wasn't that impressed with the actual writing/editing in DA. It just felt like Brown was extremely reliant on certain words ("bedlam," "meat") and used them repeatedly throughout the book. It became a distraction. I didn't really have that complaint of IG.
Dark Age is a good book. Definitely some fun stuff in there. But overall, I think Iron Gold is better by a comfortable margin. Okay that's my hot take.