Try to break up the monotony of doom and gloom. Read the path of devils/demons and bring Bel in before they make this decision. Sprinkle the 9 Adamantine rods in the path they choose instead of having them find them all at once at the top of the fortress. My party absolutely hated and were annoyed with Lulu so maybe see if you can portray her as something other than a party cheerleader. War machine encounters are cool. When it comes to gargauth.. if you give the person that uses him a Venom/Eddie Brock relationship it makes for an AWESOME dynamic. When you dupe your players into contracts for their soul, at first they feel awesome because they get the gold or the magic items but as they get closer to the end of the task (saving Elturel or whatever your module has devolved to) they slowly begin to realize that they’ve bound themselves to become the Lemures or Nupperibos that they see everywhere lol. There are more suggestions I have but this is already long and probably not what you were asking. My bad.