It's mind blowing how a handful of people think I've done zero research on this and am flying with big open questions by the seat of my pants.
https://www.ssa.gov/international/Agreement_Pamphlets/documents/Spain.pdf
See the chart on page 2 or 3. I will be self-employed and residing in Spain.
As far as mortgages go, by the time I were to apply for one (if I even do) there's a good chance I (or, at least, my wife) will already be a Spanish citizen. But, even more so, there's no financial discrimination at all. Just different rules, as there probably should be. And, under any circumstances, you'll pay less for a mortgage in Spain than you will in the US.