It's actually a little easier at $6k. In 2017, you need $1,300 of earned income per credit up to a max of 4 credits. So that's $5,200 to earn the maximum number of credits for the year. Every little bit helps, right?
That also means it's not all or nothing to hit the $5,200 annual cap. If you do enough freelancing/online business to make $1,300, even if you only manage to hit that threshold every other year for the 20 years between when you turn 40 and when you turn 60, you'll have 10 credits (2 more than you need). While you would have to pay 15.3% in self employment tax, you'd be paying 7.65% in payroll taxes anyway, so the "extra" tax from being self employed is only half as much.
You don't pay FICA on ROTH IRA conversions. You already paid SS and medicare withholding on the money that was contributed to your traditional IRA/401k. That's why you generally cannot contribute 100% of your paycheck to a 401k, you'd end up owing your employer money for payroll taxes.
Depending on where you move or where you worked before you worked in the USA, working overseas may also count towards your social security credits (most of the countries where this would apply are in europe).