I'd offer something like 10/$330... front load the first four years to ~$40~45M per year, and then give him an opt-out. The remaining six years would be ~$25M per with opt-outs every two years, but with very significant incentives that could bring the salary back up to the $40-45M range. If he opts-out, Sox have an option to bring him back for two years at $45M per.
It gives the player a lot of security, and a lot of flexibility over his situation, and the contract would have a lot of possible outcomes, such as...
4/$180... Yamamoto is really great for four years and opts out.
-The Red Sox can exercise their option, and are guaranteed to have Yamamoto for another 2/$90... but if he gets hurt/becomes ineffective, they'd be on the hook for 6/$190.
- The Red Sox may decide at this point it's not worth the risk of hanging onto Yamamoto, and pursue other players in free agency. Yamamoto, who would still be 29, could probably get another 8-10 contract here.
Yamamoto is a solid pitcher, but not outstanding. He does not meet any of his incentives.
- After the first 4/$180 - he can either opt out, and try to beat a guaranteed 6/$150, which he may be able to do...
- ...but if he does opt out, and then has a resurgent 2028/2029, he's leaving up to another 4/$180 on the table if he's good enough... so a potential 6/$230 versus whatever the market offers.
Guarantee of 10/$330... but if he's a mostly healthy Pedro, then it's 10/$450, and no one will care about the money. Definitely not my opening offer - but if it takes something like this to get Cohen/Steinbrenner/Ohtani to piss off, then so be it.
And perhaps a player option to defer $x,000,000 of his salary per season - under pre-arranged terms (which I'm not savvy enough to even try to come up with on my own).
Throw in some other stuff. Five round-trip charter flights from Japan to Boston per year for whoever Yamamoto wants to fly in or out. Bring in a Japanese chef or something. I don't know what else. But whatever it is, do it.