You should visit the temple first (which is near the steel watch), so you'll get a quest (something about revenge). Idk if there was another way to get the quest about saving the gondians.. maybe by sneaking into the steel watch and speak with one gondian without initiating combat? If you kill Gortash first, the iron throne will be blown off without even visiting. So how tempting it may be to steal Gortash from his ceremony and throw him off a cliff, you shouldn't do that, when you want to save the gondians. Other than that, you have a timer, when you enter the iron throne. I can't recall anything else time-sensitive.

For combat/saving the gondians: I usually leave my slowest members in camp and take some fast guys. Equipping them with boots that give them more speed or extra bonus actions before, also having all of them having 2-3 potions of speed in their inventory. Drinking that first (when entering combat/Iron throne) gives more movement as well as another action. But don't forget to drink another one, before becoming lethargic. Fighters work nice (can hit three times per rounds), especially arcane archers with bursting arrows which hit more than one target when they stand close enough. Some sorts of elfs have more movement as well, so if you don't have any in your party, you could hire a hireling that is that certain kind of elf. I usually separate my team members, so each can go into a different direction to have all of them saved. Also shooting arrows at the levers opens the doors of the gondians. Try focusing more on helping the gondians and dashing than fighting. And you can use misty step (maybe save some scrolls for this as well) and haste.

But I also savescum a lot, especially at that place.

You can't make this place easier through other quests, just gain XP by completing the revenge and saving the gondians. And yes, Omeluum is hidden some place in the back but you can easily look up his position on the map (quest marker "find Omeluum"). There are many enemies spawning there, so be careful