Setting aside anything else, doesn't the geography
In 1783, the Spanish PM Count of Aranda predicted that the USA would not only survive, but thrive, become a great empire of its own, expand across North America and inspire the spanish colonies to become independent. As such, he suggested to King Charles III to divide the colonies between his sons and turn them into different kingdoms that would be tied in a web of alliances and look up to their Spanish counterpart as their superior, who would take the title of Emperor, with only the islands retained under direct Spanish leadership - 80 years before the first British dominions.
The King responded that Aranda was exaggerating and that this kind of reforms was not needed. A century later, Spain only controlled the insular parts of its American empire, and soon after lost them to the Americans.
So, What if King Charles followed his PMs advice and divide the Spanish Colonies in different kingdoms with their own national identities that would be under Spain's sphere of influence?
What would these kingdoms be called? Would this lead to a different scenario in the upcoming French Revolution? Would it allow Spain to retain the title of Great Power?
Setting aside anything else, doesn't the geography (physical
and political/social/demographic/intellectual) end up in a "eastern and western empire" sort of situation? Or "every satrap goes to sleep dreaming of how to become a king and wakes up plotting just that?"
Other examples are the various (sub)Napoleonic kingdoms, the various and sundry personal unions and dual monarchies, and even such Twentieth Century conglomerations as East and West Pakistan.
My point is, such constructs tend to be very brittle, and simple demographics would suggest that at some point, the "Americans" would have little desire, interest, or willingness to be governed from afar, even with whatever level of local control/devolution is allowed by the Spanish Empire - which, considering the basic delta is a series of junior kingdoms, is not going to be great.
As an example (and, obviously, these numbers could vary tremendously based on the ripples of this attempt at empire-building in the 1790s, but it's probably about the earliest one can come up with these numbers) but by 1900 (historically), Spain's population was (roughly) ~21 million; Mexico's was 12 million, Argentina a little less than 6 million, Columbia almost 5 million, and Peru almost 4 million. Chile and Venezuela were both almost 3 million, Cuba and Bolivia each about 1.5 million, and Ecuador, Salvador, Puerto Rico, Guatemala, and Uruguay each about 1 million; the rest of the "Spanish" Latin American republics together were probably at least 5 million people.
So call it 2-1 in demographics in favor of the "American" Spanish Imperials vis a vis the "European" Spanish Imperials.
That is not a stable situation.
It's an intriguing idea, but there's a long distance in the political science spectrum of empire and federations between Aranda's proposal and even the dominion status that (say) Canada enjoyed after 1867 - and which came about, really, because of the realities of the United States.
Considering that Canada didn't even complete the federalization process until 1949 (Newfoundland), it seems to expect a lot from the Eighteenth Century.
Given everything else that is to be expected to happen in Europe in the late 1700s and early 1800s, it's entirely possible something akin to Brazil's "imperial" history follows, in one or more of the "junior kingdoms" all the way through to republics...
Best,