You need to focus less on pay decide what kind of lifestyle you want in both the short term and long term.
Because let's be honest--figuring out how to live on $2.1 million versus $3.6 million is a very First World Problem. Anyone with reasonable budgeting skills can find a way to survive on either pay.
Are you certain you want to spend your career at a major airline? As in, you definitely want the lifestyle, for better or worse, of flying for a major airline? If so, it sounds like you better get crackin' on those regional airline apps. If you're only after higher pay, I'd stop and ponder for a minute.
I'm more of a "nothing in life is certain, gotta enjoy life in the moment" kind of guy, so I'd take the King Air gig. Life is too short to be constantly chasing a dream rather than living in it. Plus, I don't care about airlines. Sure, I'd consider going to a major if I find myself in the right place at the right time in the future, but it's not a goal of mine.
Also, I wouldn't worry much about your age. You still have 33 years until retirement. You could burn holes in the sky with a King Air for three years, jump to a regional for ten years, and still have 20 years at a major, for goodness sakes.
Good luck figuring it all out.