Actually, I don't think it's actually all that hard to say "History went basically the same"
Ok, so, every really big dragon is explicitly noted to have been bred to that weight by humans, and, well, that makes sense, given the food requirements we see for heavy and midweight dragons there is no way they could function in the wild. Additionally, it's explicit that in the Mediterranean Rome was the first to tame Dragons. So basically, everyone is dealing with dragons that are probably smaller than Courier Weight who do light raiding. Herding is a bit less profitable because Dragons but farming is basically unaffected, this might actually be part of why Agriculture starts up in the past, and part of explaining why people live in big ole cities. But eventually Dragons do like the megafauna of Africa did and figure out how to not die to humans(even firebreathers don't have enough range to stay out of projectile range and Elephants/Mammoths and what not got shot down in the primordial past, so would Dragons), and that's mostly "stay away". So, again, maybe some light raiding when the Dragons have bad years but it's not a big deal. Rome is the first to tame Dragons in the area, that means Persia didn't have Dragons first. So basically everything in European History goes basically as per normal. Even that thought of "Ottoman Dragons at the Siege of Vienna" just goes "So they were kept occupied by the Polish and allies dragons while the Winged Hussars won the ground". There's some more effort for ranged weaponry that can shoot up, but again, Dragons aren't actually all that dangerous in warfare yet, they are mostly recon, communication, some assassination and a bit of specialist roles for fire breathers or acid spitters or similar. Then Gunpowder happens(specifically bombs and the like) and suddenly big dragons mean more than winning scout fights and being more resistant to arrowfire so you can pull off cooler assassinations(though, human riders still), it means actual firepower.
Now, with this idea in mind it's a bit weird that Heavyweights are so big, but it's not that weird, Dogs didn't take all that long to get bred into wildly different new breeds, and it's worth noting that the way Dragons are discussed really does read more like breeds than species. Iskierka is such a big deal because they are hoping to breed firebreathing into British lines through her.
No, my own problem with the Temeraire series is this. How the everloving fuck did the slave trade as we saw it even happen? If every African Tribe has a Dragon Ancestor how the everloving fuck did anyone ever get sold to begin with? And if the Europeans went and kidnapped people(which they didn't IRL for reasons that still very much apply only moreso) then how did it take so long for the Dragons to find out and react as Dragons do when their people get taken? Remember, Dragons live a bloody long time, war between tribes probably ends with prisoner exchanges unless one tribe is totally annihilated. So there's not the argument of "unrelated Dragons selling prisoners of war" because they want their own back. If every slave had to come from annihilated tribes that's just not sustainable, that very much doesn't lead to the status quo we saw IRL.
Still, I do like Laurence and Temeraire and most of the characters close to them... even if the plot/setting is damn stupid sometimes.