Now that the Congo is once more back in native hands, what are the opening moves of this state?
Well, domestically the Congolese army and their mercenary auxiliaries are (after looting the hell of the colonial offices to pay the troops) going to be in siege mode, fearful of any potential invasion from the Europeans, even after vague reassurances about territorial integrity. They have too many contacts with Western Africa to think otherwise. This means that their main aim is to ramp up the army size and military establishment dramatically.
This is far more doable than it actually looks due to a couple of advantages the state of Congo has.
Perhaps most critically is is that all that wealth from sales on raw materials that would otherwise be flowing out of the Congo can now stay at home and be used to pay for the Mamluk Administration and whatever projects the government wants, and that's a lot of money, even after the low tariffs and concessions. Its also diverse by default, with ivory, cloves, rubber and copper already being major commodities in place by 1895 prior to the revolt. Congo being quadruple the size of France helps with that.
Furthermore is that this new Caste is, for the standards of the day and place, well educated and disciplined. The Belgians essentially imported the Prussian military school system into their colony, and so the army won't be short on soldiers with the gifts of literacy in French, basic mathematics, basic logistics and a bit of political science. And the discipline of the army should help curb corruption a bit. Initial numbers would be small due to the limited numbers of teachers available, but as the years go by this will increase exponentially.
As well, there are 2 more benefits that the Mamluk government can use for its modernization plans. The first is that an early end to Leopold means an additional 10-13 million people aren't killed, and therefore available for the economy. In the short term, this is a greater pool of manpower for army recruitment and working the tribute, but in the long run as the economy grows increasingly more complex, this means more tax payers.
The second is primarily economical: native Congolese are no longer subject to colonial restrictions on their economic autonomy. Rather than having the riverine trade be dominated by Europeans (as natives were 'discouraged' to join in), once more the old system of network can start up, which thankfully, its still early enough that plenty of people remember how the old ways work. Furthermore, plenty of new jobs opened up. No worries on Leopold's stupid restrictions on how natives can't work in X land because its state property and such forth. This opens up massive amounts of jobs and economic activity, and creates links with all the other Colonial states to trade/smuggle. Even a small tax on plantations would be a massive boon to the state.
And now for the bad news that stack the odds against Congo:
The Army's greatest strength is also its weakness. That it is /the state/ means that it will need to be everywhere, much like the state is on paper. In other words, the Force Publique will now have to constantly be split, as administrators that would otherwise be officers are going to be reassigned as teachers, bureaucrats, managers and so forth. This shrinks available manpower (initially), and increases a reliance on mercenary forces from Zanzibar (or Nigeria or Sudan). Such problems will be rectified in about a few years or so, once the new batch of recruits are ready, but for the time being its bad.
Additionally, the economic situation is terrible. While yes I've previously said that Congo will have a booming economy due to the end of colonial restrictions and siphoning or profits, but at this point its starting from nothing. The Congolese economy was at this time basically a glorified tribute system. Trading posts would tell chieftains to pay taxes of either [insert material here] or men. There was little coinage and truthfully, little trade outside the rivers. This means taxation for the state is going to be virtually null, and the budget will have to come from either taxing people in cities, or customs revenue from European riverine trade, which will be put under pressure by outside forces to be as low as possible. That leads to the problem of smuggling and people not wanting to move to cities (initially).
Oh, and of course, that tribute system isn't going away. Rubber, Ivory and Minerals are what creates the wealth to print the money that pays the troops, so while the state isn't going to be as evil as the Belgians in resource extraction, its not going to be nice. Expect comparisons to Brazil or other Latin American countries in exploitation. Furthermore that tribute is going to be prone to abuse by local authorities, and there's not much Boma can do about it. Congo's just too damn big. But I guess the addition of that extra 10 million people (and growing, once basic modern medicine and farming start steadily flowing in) will be able to make up for it.
A silver lining is here though: Congo will have plenty of surplus labor that can further aid the supplementing of the economy. The birth pangs of industrialization, combined with agricultural improvements and half the population not dying in the 1890s leads to a very large labor force. Assuming European corporations don't hollow out the state and start running their own fiefdoms.
Oh and finally but most importantly is the problems stemming from the political sphere. Congo is a literal army with a state glued together with bullets and feudalism. Expect the utter domination of the state for decades on end by a Soldier caste that reigns supreme over hundreds of ethnic groups. Military dominance will mean the creation of a deep state just like in Egypt or Argentina. This privileged group, like all groups with power, won't be so keen to share. Expect conflict with the middle class and urban poor in a few decades as this new group of educated individuals, with radically different notions of religion, culture, autonomy and so forth clash with the establishment. But that's for later.
Now, what does all this add up to?
Ok, so this means we'll be seeing initially (i.e in the next decade) the state ramp up drastically its administrative-military complex, sowing the seeds of a deep state. Short term this means any infrastructure improvements will have to be private things, not state things (doable but small scale as it'll take a long while for anyone other than chieftains mobilizing the local tribe's resources to pay for whatever or missions setting up schools/roads/etc).
Expect the troops numbers to swell to at least 50-100,000 men plus whatever auxiliary forces come about by 1910 or so. And for some sort of homemade military industrial complex to grow in the Congolese cities to feed this army. Buying supplies is expensive and again, this is a caste that has people who have been to Europe and know how things work. Some European captain of industry is going to have the bright idea to set up shop in the Congo in exchange for some of that mineral wealth. So small scale industrialization to feed the vastly growing army, clustered around Bas-Congo and rivers that serve as the arteries of trade.
On the economic side of things we see a massive expansion, if only because there aren't 10 million people brutally dying, the economy is just /that/ simple, and the gatekeeper state isn't implemented. Short term, this is great, but mid to long term, we'll get massive problems.
Politically, all thoughts of democracy are out of the window. The guys who will run this Pseudo Republic are a closed off, semi elitist caste of military troops united not by ties of ethnicity or language, but by a common religion and common institution that they hold very dear to themselves. While yes, they'll still have massive ties to their villages and families, they will, like US military brats, see themselves as fundamentally different from the common people (and to be fair, they will be for the next few decades). This is something that happened historically actually.
The political system will be Mamluk soldier/administrators running a semi feudal system collecting tribute and taxes from the local statelets and tribes. Don't expect common laws other than a rationalization of business laws and common tariffs. In other words, this Congo is more like the bastard child of the EU and Burma than anything else. When the new generation of homegrown Congolese businessmen, doctors, industrialists, and so forth, who send their kids off to Europe for education come back, this will change. But for now it'll be pretty rough. Still leagues above OTL if only because the Free State was just that awful.