That he did, in ASM#49/#850:
https://i.imgur.com/WWPC4JH.jpg
He was blatantly surprised, and annoyed, when Peter pushed him off the submarine:
https://i.imgur.com/MSs7d5S.jpg
(ASM#50)
Also even if we assume that Norman delayed the whole thing of being cleansed by Sin Eater to have fun as Green Goblin and **** around with Peter before being cleansed (Since that's the only way to explain why he'd try to delay it so much), this whole plan you mentioned
completely relies on Norman thinking that Sin Eater's power wouldn't work on him, somehow, with absolutely no basis for it, and be right... And it's just garbage writing for that to be the case without a proper explanation, and would be on the same level of crap writing Go Down Swinging had with that "Goblin Serum makes the Carnage symbiote magically immune to fire and sonics!" nonsense.
Best way I can think of this working is that Norman at some point took the Goblin Serum off-screen and, he'd count on that making him go back to his "normal" (Which is already pushing it), but considering that Sin Eater makes people legit regret all the evil **** they do, he wouldn't take it again willingly.