It's not exactly ntr. There will be Amber-Henry scenes for example if you choose to give him advices and leave Amber alone from the start. So technically it's not ntr since she's not your gf but in a way it is ntr cause in other routes she is your gf.
Bruh, ngl that's kind of a dumb move. Not only is a pointless waste of time but you wont get the ntr fans from that since its "technically not ntr" which cucks don't care about, also harem crowd is gonna be pissed... my advise if you want to go that route, do a text only kind of thing and don't waste development time on side characters no one really cares about.