Also not wanting to start a NTR debate, but thing is the NTR genre is about only a few things. Basically NPC rapes a female into his submissive slave with a weak MC that can not stop him/her and will never face any consequences or danger during their actions. (For rape can read blackmail/drugging etc).
Now besides being pretty boring if played a few of those games it is not a real problem for a full NTR genre game, since that is what the fans of that genre actually want and love. I actually doubt the fans would like a slow burn NTR since they do not immerse or if do, they immerse with the NPC most of time, unless have a love for being cuckolded. I could be wrong though.
A problem arises though when make a game with both more romantic love routes and a NTR genre route, namely character integrity and speed of developments. In this game that is partly taken away by the use of magic, which in a way excuses the female NPC's for their behaviour and makes it more acceptable that the females react different in the NTR route as romantic route. Still the female characters should make sense on both routes and dev should be aware that if there are already gang bangs on the NTR path there will be grumblings if on the non NTR path MC is just allowed to hold hands or kiss the cheek of those characters. It is not an easy thing for a dev to balance those two different paths in a game.