Just got (kinda) rich--what do I do?

I (M54) got an advanced degree about 20 years ago-the kind that is known to make folks a lot of money. For various reasons, some my own doing and some not, it didn't really pan out for me, and while I've been doing OK the past few years, I haven't had a lot of disposable income.

Surprise! Just as the world is falling apart, I got my dream job. My salary more than doubled overnight. I love everything about the job, and I hope to keep it until I retire.

So what do I do with the extra money? I was thinking about putting an extra 1K/month toward my mortgage (4.75%, 18 years left on it), which would pay it off 12 years early. But then I realized that money would probably serve me better if I just put it into an S&P index fund, right? S&P returns like 9% historically, and I'd only be saving 4.75% on the mortgage. I have a Schwab account with very little in it; should I just start loading it up?

What else do I need to be doing? I have a 10yo son, definitely going to look into one of those college saving accounts. I have retirement accounts from current and old jobs, and will probably receive about 500K when my dad dies (likely in the next 10 years).

Honestly after the initial rush of that first huge paycheck, I'm a little overwhelmed with options. Any advice is appreciated. TIA!