Just to use all the money in your credit cards to buy some BTC and then before the grace period pay them back and repeat. If I am not wrong, this should give you a very nice profit say for a total amount of $20k between 7-8 credit cards plus good credit and all the cachback and points each CC have.
Worse move ever.
You say 20k "between" 7-8 credit cards. That leads me to think your credit line is around 3000$ per card.
That means you have either a low credit score or if you leave outside the US a low annual income of around 12k$.
Not a smart idea.
If the
BTC price doesn't go up as planned the interest will deal a blow to your finances.
Well, I am always opposed to credits and debts but maxing your cc looks like the worse of all.