Congratulations! Praise the sun!
I’ve been holding off on watching Prepare To Cry until i beat the game, but that’s supposed to be good for lore. Hbomberguy had some good vids but that might have been dark souls 2 and bloodborne.
For the second playthrough… you may have done this already on your first but there’s a thing you can do in Anor Londo that is significant lore-wise and introduces a new boss. And there’s another primordial serpent that you can meet if you go about the game in a different order, again significant lore-wise and think it has an impact on the ending.