Question about endgame

So i pretty much finished the game or so i assumed

I just lerned about the izanami fight/duengeon and i wanna know do i have to max out everyone ? Do i need to be in ng+ for that?

i read online what to do ( speak with people on the final day at junes and stuff) but i just kinda wanna hear from multiple sources so i dont screw stuff up

I beat the hollow forest and saved marie and got what i assumed was the true ending ( until i heard about the izanami fight)