I am very similar, (though not self employed, and depending on your definition of 'decent sized' I probably have a smaller stash) and ,retirement accounts aside, am planning on starting with the S&P 500 index, then either go with something with higher dividends, or maybe just the total stock market index. DRIPing everything of course, until in FI. All invested via Vanguard, of course.
I'm sure there will probably be other takes, and people will probably want more info, but I think "hey, I've got extra money, where's the best place to start?" will pretty much end with the s&P index or total stock index.