not tough. there's a lot of POD's that lead to a stronger Spain. there are a lot of inferior kings for 200 ish years in both the Hapsburgs and the Bourbons. Pick your year. 1650, 1700, 1750, 1800. a hero (or even a competent) sperm latching onto the egg of the future king will make a huge difference.
the entire second half of the 17th century hinges on Balthazar dying of appendicitis. simply having him live butterflies at least one war. likely butterflies the abomination of Carlos II, the war of Spanish Succession, changes the entirety of Spain in the 1700's and beyond. Balthazar doesn't have to be a good king. simply being sane changes everything.
Just as the US is almost ASB for good luck, Spain is almost ASB for bad luck. the only thing that makes either believable is that they both happened. swap the ASB around. good mojo for Spain, bad for the US.