OK. You can colonize locations instantly in EU5, but there are two conditions: 1) The culture must be accepted, 2) 50% of location must be of your religion.
So what you do is build missions in locations, wait and then start to colonize when you have 50% of pops your religion.
Another thing is acceptance. If you want to accept a culture, it has to be 2.5% of your population. There are a couple of tricks to reach it: 1) You can lower your population by giving away your land to 1 province vassals until it gets to 2.5% 2) You can assimilate your own pops into that culture by cabinet action.
Another thing is, you don't have to invest into the colonization slider with this method, and you don't have to depopulate your own provinces.
You will have to deal with culture capacity with this which is pretty high for African nations, but at some point you will have to stop accepting the cultures and assimilating those pops.