But before Portuguese control of the route...
As other people said, they were well in their way to control the road, and already did for African shores scout and mapping.
But Spain had a chance IOTL, why not in this scenario?
Spain managed to get "dibbs" on America because Colombus expedition was crazy : he believed the earth was shorter than everyone said (and that we say still today). Portuguese reaction was more "Okay, you do that", than really feeling they passed a chance, and without the conquest of Mexico and Peru, Spain would have gained the chance of wasting ressources (I'm a bit unfair, sugar cane plantations in Antillas would have been a bonus, but nowhere close of Indias trade).
See, Portugal was a bit like America and its space program, when Spain was an USSR with half-assed programs. One says "Let's go for Mars, at such date and in such conditions", the other argue that you could cut short by going trough the Sun.
Talking about Spain especially, because were very interested in Christianization of conquered peoples.
There's a confusion : Spain, as a state, wasn't interested on conquering people, but spanish nobility was interested on conquering lands and the spanish rulers were interested on gold.
Talking about state imperialism is a bit of an anachronism as we're seeing several social layers with different interests that could be as well divergent if not opposed.
Lands in Black Africa needed an awful drain of ressources with the current technology. Gold was a think, critically in Sahel, but the tentatives of conquest or at least disruption proved wasteful as well and nowhere as close than Indias trade.
For Christianisation, it depends really and is not the same as conquest : OTL Portugal took over Maghreb coast and places in Indian Ocea but couldn't be bothered to convert people living there. On the other hand, african leaders as the Kings of Kongo converted themselves without (direct) control.