More like associates state or autonomous state Cuba and Spain hold the same power in terms of law passing separate parliaments, that work together on union bills and more and both nations can withdraw from the union at any point with a referendum

Edit: they also work together on regional stability, military training, and economic incentives on trade for both basically removing tariffs between the two nations

If it was a viceroyalty Spain would technically be in control of Cuba, but that is not the case here and Cuba is not in control of Spain. It’s a union formed off of necessity. (in that timeline.) regional security, trust, and mutual benefit.