If you're maxing out 401(k)s and HSAs, the only thing left is the Roth before you hit a taxable account.
Consider whether you'll be better served in the long run by freeing up cash flow by paying off your mortgage or student loan. People go different ways on this, especially since it's at such a low rate, but you definitely have more freedom in life when you don't have mortgage or student loan payments.
I like to compare it to borrowing money at 3% in order to invest in a taxable account. Would you do that? It's basically the same thing when you are putting money into a taxable investment account while you have a 3% loan outstanding.