You're forgetting that it's taxed at 50%, probably closer to 55%, so what he takes home is $350 million over his 10 year contract at 50% taxation. That's $35 million per year over the next 10 years.
When you consider that professional major league players usually only play to the age of 40, that means that in 11 years from now Shohei Ohtani will be 40.
If he lives to be 90 years of age and he's almost 30 right now, $350 million after-tax dollars divided into his last 60 years of useful consciousness is only $5.8 million per year.
Gross income means nothing except to the ego. After-tax dollars is where the rubber meets the road.