Well that Spain was irrelevant in all periods after its golden era is just not true.
The golden era of Spain was from 1492 (conquest of Granada and discovery of America) to 1643 (battle of rocroi and the end of the military hegemony). During these 150 years, Spain lost no land battle despite being at war constantly with almost everyone and everywhere (take that for achievement). Its naval force was feared and respected.
After this period, I like to think of Spain as a giant-slayer dwarf:
In 1700, Spain was so relevant that its succession war to the throne caused the first great war of Europe (https://en.wikipedia.org/wiki/War_of_the_Spanish_Succession) with the intervention of Portugal, France, Austria, HRE, Hungary, Britain, Dutch Republic, Savoy, Prusia, Baviera, Cologne, Liege etc.
In 1775, Without Spain intervention, the USA would have probably lost its independence war. Spain invaded from Louisiana and overran the British Caribbean.
(https://en.wikipedia.org/wiki/Spain_and_the_American_Revolutionary_War).
in 1808, Spain was the first country to defeat a Napoleonic army which until then were thought to be invincible. Meanwhile other actors -such as Prussia and Austria- were defeated in a matter of days/months. Moreover, Spain was a very important factor for the fall of Napoleon. The spanish ulcer -as he called the Spanish war- drained energy, men, equipment and funds from many other war scenarios which ultimately sealed his peril.
Not only that, the Spanish army despite suffering many defeats did not stop fighting and regrouping again, and again, and again, and again and again until the French armies was expelled from Iberia. Even the civilians rose to arms, fought, and harassed the French. This did not happen in other "important" actors of the time.
in 1936-1939, Spain was the first country ever to defeat a communist regime. Once defeated, Franco send the Spanish Blue division to fight against communism as a big F*CK you MR. Stalin! Of the 45000 men of the division, 22000 died in action and managed to neutralize 50000 soviets. Even Hitler considered the division as equals to the best German troops, and Spain as the only tough latins (https://en.wikipedia.org/wiki/Blue_Division).
Other examples can follow if you wish.
So please, don’t tell me Spain was not relevant after 1650.
Besides, England knows better (https://en.wikipedia.org/wiki/War_of_Jenkins'_Ear).
Changing the topic, I once wrote some ideas to improve the gameplay of Castille/Spain for EU4 in the MEIOU forum. Reading about upcoming expansion, I really do think these may come handy, especially for a country as important as Spain for that specific timeframe.
And that not counting the Reconquista, Roman Hispania (where some of the best emperors where born), the mighty Caliphate of Cordoba...