Exactly she needs somebody that can take on Yichen, it's definitely not gonna be the mc within the next 100 years, and yeah she could take He Gang body whenever she wanted but probably not in a condition good enough to take Yichen on.She said she can't enter the realm with her real body because of Tang Wei's protection.
She's also already made an Avatar from materials in the poma world which is stronger than anyone not named Yichen.
And if she wanted He Gangs body she'd just take it. She has no reason to do it in such a roundabout way.
So my theory is she manipulated him into preparing his body for possession so she can use her full powers. It might also be a backup plan if the mc doesn't join her.