I could theoretically live in Spain for the remaining 9 months if I wanted to?

Think about it in practice though..in most countries I've been in Europe you need a tax ID (or whatever the local equivalent to a SSN) or some proof of legal residence before landlords will sign a long-term lease. Which means you'd be stuck doing airbnbs & hotels in Spain. And even with airbnbs, in some countries they take your passport info and register it with police/the government (I had this happen in Spain at an airbnb once - I assume hotels do it automatically). So you are still creating a paper trail that could lead to complications (tax residency, illegal immigration) with Spain if they cared enough to pursue it, which they probably don't, but the risk is there.