The dude is not signing away 3 free agent years for a fraction of his earning potential. Maybe you get him to give up one year on a 3/50, (say 5/10/25) so as to push some money forward and give the Sox a bit of a break on a free agent year, but he is not going that cheap on a mid length deal. 5/75 would look some like 5/10/20/20/20 which is absurdly low to give up three seasons where he could be looking more like 5/150 after the next two years coming at at like 3 and 10 million through arb if he wanted to wait for his powerball number. If he actually gets through the next two season relatively healthy he might be able to push that out to six seasons in the 35 to 40 million per, range.
He has no reason to sign a cheap and longer extention. He sees pitchers going half seasons for a lot more money than this. All he has to do is make it through the next two seasons as he already had his first arb year. In your version. He gets a free agent payday in 2 of 7 years. in his, he somewhere close to doubles he $$$ if he is healthy, even if throwing half seasons he has the Glasnow contract as a comp.