We don't have the resources to like, have multiple different endings and storylines/narratives.
You don't have to do that. No one does that, not even triple-A games, not even the old ones. Save for a few extremely rare exceptions there are usually two kinds of "multiple endings."
1. You make a choice at the very end; it creates the illusion of multiple endings if your choice was informed by what you've learned throughout the game. Deus Ex, Mass Effect type games do that. There is no long ass script that has to remember everything you've done. This is best for stories that pose a central question.
2. There is a single value that is altered positively or negatively throughout the game by player action. Best for stories where you want to juxtapose two different types of character behavior. But you can also make that very complex and have 2 games in 1 (Undertale), or very simple and have it only change the ending (Dishonored).
I don't know what this story will be about so I can't tell which one is best for you (probably 1) but neither requires more effort than you've put into it. You don't have to be afraid of multiple endings.
In any case, I have a different type of question. Is the party set or are you open to adding more party members later on? Because there's a kobold thief with nothing but pillow-humping to do and we know every proper adventuring party needs a dedicated rogue/thief.