Let's say I don't get a NIE, which seems the best option, and I don't open a Spanish bank account - do I still need to not use my cards? It's a pain to get out of the country and come back with cash on a regular basis...
I understand, but at least try not to use your card every week for more than the 6 months, otherwise, if they investigate your card, and they do it, they will not be able to prove that you've been in Spain for more than 6 months.
The bottom line is that you need to make sure they can't find a way to prove that you've been living in the Country for more than 183 days, so no electricity bill in your name that shows that you've been there for so long, no water bill, no card purchases or ATM withdrawals, they have access to all those things.
Just be smart about it, you can have enough cash well hidden at home and use that cash for several weeks without having to go to the ATM machine every few days, I'm sure you understand the concept.
And you can get a NIE if you need to, that's not the issue, just don't leave evidence they can use to prove you're living in the country for more than 183 days.
There's no need to get paranoid, there's a very small chance of them investigating you, but just be careful so you don't have to worry in the event of an investigation, remember they can easily charge you, then you would have to go court and prove them wrong, to give you an idea, about 50% if the cases are lost by the Spanish tax authorities, so a lot of people end up being charged and then found not guilty, just be mindful of the traces that you are leaving behind, is not difficult at all.
Hope that helps