The endless are 100% implied to have sapience because an AI program created to eternally preserve a persons memories would be unable to take steps towards ending its own existence. Cauchia is definitive proof the endless have a degree of agency
The endless (via cauchia, Otis, krile’s parents and the proposal NPC) are all fully aware they are endless and understand how their programming works but also have shown to act beyond it, particularly the proposal NPC who states he has been spat out by the system before and never made a move for his finance but chose to do it this time because he is unsure of how long the existence of the endless can last (a common thread amongst the endless in that they understand that their existence isn’t forever), in that scene he breaks out of the implied temporal lock that exists amongst the endless. The NPC that makes the game for you in yesterland and changes his age when he realises that his childhood form doesn’t make him happiest is another example
The endless may not be human but they are 100% sapient, they aren’t the people they were based on, they are different entities that use the memories of the deceased Alexandrians as a foundation