Honestly, the endings aren't that big of a deal. Since you've said you know the endings via your mouthy informer, I don't feel too bad saying this, but they're very much like Dark Souls's endings in that it's just up to what the player wanted to do with their personal game world. Lore-wise the endings only have two implications, and the same two implications as in Dark Souls; do you prolong the inevitable death of the world or do you doom it for your own personal gain? Whichever one your "brave hero" picks is entirely up to you as a player, and no other actions taken in-game can determine the outcome beyond your personal roleplaying preferences.
As to what can influence those decisions and make you decide if the world is worth saving or not, you say you're a lore nerd so I assume you know this but I'll type it just for emphasis, you need to pay attention to the world around you. Read item descriptions. Read into the emotions behind characters' dialogues. Study where things are placed and how the environment is laid out. Find the clues and reach conclusions. You're at the Dragon God, so you've played through all of Stonefang, right? Think of things like this:
- Still spoilered for your convenience:
Based only off of these three clues found in the last bits of Stonefang:
1) The Flamelurker's arena is covered in the skeletons of dragons.
2) Shortly behind these is a temple housing the one and only remaining living dragon, who is pinned down and ready to be detained at any moment.
3) The Stonefang citizens had developed a weapon, the Dragon Bone Smasher, specifically to fight off and defeat dragons.
Did the Stonefang citizens dig too deep and uncover an ancient secret best left forgotten and developed what they needed to survive against this threat they accidentally stumbled upon? Or did they discover the dragons and develop techniques to slaughter them in a greedy attempt to harvest their bones for crafting materials, only to have the leader and most powerful of the dragons drive them away whereupon they sealed said leader in a prison?
There are other outlooks, of course, but how you interpret things like these through observation will affect the appropriateness of the ending rather than the actual contents of the ending itself.