Someone already explained how to access ''new game+'' and access the pink hair route on discord. It's actually super easy. I'm gonna copy/paste it here:
Load a previous save (a save before you reach the ending) then skip forward until you get the ending (With the ending video and everything)
Then immediately after the game send you to ''main menu'' , click on ''new game'' and skip forward until you reach the partner choice of challenge 3. When you reach it then click on Lamda and you will be able to access her route.
btw I agree with your suggestions I hope the dev adds LGBTQ+ characters in the next update. I know there is an outcry about gamers not wanting diversity in games but I think visual novels and especially this game will benefit from adding a gay guy or gay girl in it to mix things up.
Representation is very useful for games in general imo