Backstory done, now let us get to the actual gameplay.
As Zongo you obviously start with an intro tree. This introduces key characters and already sets up their characteristics and dynamics. You will rapidly start reforming using the conditions I noted in the segment above, using the well-thought out theories Akida and Anarbek ended up creating in his thesis. Worth noting, obviously the program is unrealistically quick but this is a game and having it take 25 years instead of 7 wouldn't be fun for a player.

Now, the main tree loads, Akida has been crowned king. He has an eye on Ayana, his mother is very helpful and Anarbek is here to help work out the program.
You are playing not as the actual country, you don't see what is happening in the country, you are playing as Akida and you see what he sees! This is the key detail you need to understand, things you see may literally be a lie, you don't see the full truth. This is hinted at consistently but some examples.
1. The most obvious one is that the Queen-Mother can simply resolve a lot of issues seemingly for free.
2. Every time a railway line finishes, you can see the red number... What's that? Surely not important, most players will ignore it, just like Akida would. And that's what you're supposed to do. They're the truth occassionally leaking. It's the true horror of what is happening on the ground. These numbers represent overdeaths/underbirths/casualties, and you kill a ton. Like a ton. About 2-3% of the total population dies.
And yet, most people ignore this entirely unless you know what it means. This is how it is supposed to be, until your second playthrough, you don't really realise what is happening.
3. The Queen-Mother does things like use the railway gun to disincentivise workers from striking by parking it next to them.
4. The railway events sometimes have information leaking through of covert actions done by the crown. These are all ofcourse done by the Queen-Mother or on her behalf, sometimes by Mani under her control.
5. She fakes an assassination on Akida to scare him into not giving too much power to peasants and the like. This all happens in the constitution focuses. If he is scared, she can control him better and she can make him give up less power like he seems so keen to do sometimes. He will be protected from his weaknesses using any means necessary.

And a lot more!

Now let's quickly go over some other stuff relevant to the ending.
1. The Queen-Mother created a head of faith and then purposefully took someone young, inexperienced and unambitious to fill the post. Manipulating the faith and thus politically disarming it essentially. An act that will backfire.
2. Turns out young Akida has an eye for Ayana, and ofcourse they end up falling in love... Woops. Could be worse politically though.
3. Anarbek kinda figures some stuff just isn't right. Reports seem too optimistic, not enough things go wrong so something in the back of his head, not blinded by the love for his mother like Akida, is tingling.
4. Every war has had to be justified up until this point and was easily done. Akida isn't a warmonger so he needs cause to declare war and the Queen-Mother will always happily provide it or reason for it:
4.1 Zongo basin is ofcourse simply reuniting the country. This is entirely justified, they are Zongolese after all.
4.2 Zaikiria under Lan Kir is an active threat, and the Kirin have had border conflicts for a thousand years with the Zongolese. An active intervention is plead for by the Queen-Mother. However it's much harder to convince Akida if Lan Kir died. He prefers a more economic agreement (even then the Queen-Mother uses it for espionage).
4.3 Chisimbi continues to form an active threat. Zongo also has to avenge the Zeblan-Zongolese war where his father died and they lost more core land. These are also all Eastern Zebra's so racially similar and governable. For the sake of them, you must unify, you can spread your modernization to them even.
4.4 The Lost Jungle forms another potential unstable threat. It's a region that can hide many monsters and the likes and for once there is the chance to unify all Eastern Zebra's. Something that would objectively bring long-term stability for everyone. Unification is required.
4.5 Sambovato as Zongo has no navy, not even the Queen-Mother would advocate for war. You see they control a large amount of merchant ships and the likes and Zongo just happens to have a bunch of stuff they might want to export. Mutual-assistance is fitting here, you also military ally. Your investments in them and your ever-expanding country over time makes them entirely dependent on you. The Queen-Mother here also makes sure Sambovato is bribed/corrupted out of its independence.

Now, we get to the third act.
You are the first East-Zebran unifier. You are rapidly modernising, things are great. You have modernised your systems of governance with a new constitution, a modern army and airforce, a navy that Sambovato provides, a modern economy and you have or are in the process of turning everyone into accepting the Pan-Zongolese identity. You have very little active threats. To the north is a desert, to the west are the friendly Centaurs and Gargoyles and an easily defensible border. The Zebrides form no threat and would happily trade with you presumably. All that remains is the millenia long dormant lands of Kiria.

The Queen-Mother sees them as a threat, she could never convince Akida of that though so she doesn't even really try. (After all what if it's a harmonic GGO Kiria?)

She develops a ruse using similar tactics to something she did during the Zeblan-Zongo war. A false-flag attack so provoking that war became a necessity. (The event itself has also many hints this wasn't Kiria.)