Yeah, there's been dissent on whether he knew and had the plan when he read the e-mail, or whether he started planning immediately after the energy exchange, or later on.

He knows he's going to have to counteract the dark energy in Ye Zun so he's going to need a higher light/dark ratio in his own. It's why he's so incredibly weak there at the end.
Also, I would like to inform Shen Wei that his plan is stupid, because what if Ye Zun doesn't eat him, or leaves him for last? Nerfing himself is a terrible idea! *grumbles* (And yeah, I think he scheduled future cutting sessions in his own apartment, or other places where Yunlan wouldn't spot it.)

It's why "worth it" comes out of him so readily--because he doesn't think he is worthy of ZYL (for various reasons, probably including keeping his brother locked up, letting Dixing fall into such disrepair, all the other stuff that isn't his fault but he takes responsibility for). It's why he smiles at ZYL as he's dying on the floor in the palace. He thinks martyrdom is what will make him finally worth it.
Okay, congratulations, you have managed to create a take in which Shen Wei's issues are the worst! I think you deserve an award.