A lot of this depends on how Spain is able to get a larger Spanish Guinea which will be linked to broader Spanish political and geopolitical situation. I don't think it plausible that Spain can hold all the territory without serious changes to the later war with Britain and the Napoleonic Wars. I think the idea of a more limited but still significantly larger Spanish Guinea can be had with later PODs.
One timeline that explored the concept was the Prussian on the Spanish Throne where Leopold Hohenzollern-Sigmaringen becomes King Leopold I of Spain following the Spanish revolution of 1968 with France failing to learn of the situation until it was too late. Throw in Carlist issues on the border with France in the Basque country, create a divide with France and move Spain into more of the German camp.
Find a resolution to the Basque issue and the Cuban independence movement. Have Spain get a more stable government in the mid-1800's and have them press their case in colonial affairs. For example the Spratly islands formally considered part of the Philippines by contesting the French in Indochina as well as expanding and laying claim to the wider area of Spanish Guinea from the French too. Also holding the claims on the Caroline Islands.
The timeline in question has Spain successfully bring Morocco under a Spanish protectorate, stabilise control of the Philippines, de facto bring the Dominican Republic back under control of Spain and going to war with Germany against the US to preserve their colonies. With varying degrees of plausibility.
Assuming a more stable Spain at home who brings resolution to a lot of their open wounds and successfully challenging French (and other) encroachment of their territory isn't exactly the least plausible timeline. If we assume a Spain that maintains this control over the Spanish Guinea with this kind of set up, and they don't lose it (no WWI, Spanish-German split, German victory) Spain would likely see the colony as a matter of pride and protect it from a France with whom they have beef.
It would likely not be a major concern for them and if they have control over their islands in the Caribbean and the Philippines. It would be interesting if the territory came out of colonialism having more land and development from a country able to invest better in its infrastructure. Maybe as part of some kind of Commonwealth of former Spanish colonies. Such a Spain could plausibly hold the land for a long time before letting go.
