Is there a way to spawn in a chance cube already preloaded with a specific reward? Or failing that, a way to spawn in a one use reward selector pendant with a reward already preloaded into that?

https://github.com/Turkey2349/ChanceCubes/wiki/Reward-Selector-Pendant
and if you want a pendant that only has a single use, check out the single use selector pendant

I can use the selector pendant to type in the reward, but is there a way to spawn it in with the NBT data already loaded so that it doesn't have to have to have the specific reward written in?

No, because of the nature of how the cube work currently, you cannot assign a reward to a cube

Alright, thank you for the response.