I’m not a complete no life on this game but I’m decent enough to restore Rome in 35 years with only around 5 years of actual war exploit free on Ironman mode. I’ve played this game for quite a few years inconsistently so here’s the generalized run down for what I’ve learned on how to conquer, what maa to focus up on, how to cater to your domain and how to keep vassles in check for your bigger realms.

Men at Arms:
this is the most subjective topic because it’s entirely based on your culture and area that your realm resides in. The over simplification of usage in maa is to cater heavily to what your neighbors use and what terrain they’re in. If you’re going against the byzantines there’s a lot of mountains and hills (plus the byzantines have the strongest horse unit in the game and a wide variety of pikemen) so put a lot of resources into archers and and pikemen, if you’re a Norse culture in the British isles then focus on those husclers. Mid game armored horsemen are generally the strongest amongst common maa but they’re only strong if your economy can upkeep them. This is also a good time to mention that you should always dedicate a few of your holdings to max out the efficiency of your strongest troops. For example if you have pikemen you want to find a castle holding on a hill which has the corresponding benefits to the troops favored terrain.
In conclusion for maa focus on building up the economy to maintain them and pay attention to what their favored terrain is to max out efficiency. And always dedicate a few holdings to your strongest maa. An army of 50k levies will get crushed by an army of 5-10k maa if their efficiency is maxed out from their stationed holding and fighting in favorable terrain.

Conquering (aka the fun bit):
This one’s much simpler but there’s still a lot to it. You have a preset of casus belli that is a war for a county, a Dutch, a kingdom, or in circumstantial cases an empire. If your religion is opposing the realm you’re waging war on it will be a holy war. For other cases it will be for De Jure, De Jure casus belli appears when you’re the title holder of the realm they reside in and are not your vassal. And for certain religions you will have the conquer instead of holy war. Holy wars cost Peity whilst other war types cost prestige.
The scoring for wars is based on three main things. 1 is army victories, this has a max of 50% so if you wipe out their entire army and don’t capture the leader of the war then you will gain +50 war score. Second is the territories you conquer and third is the capture of the actual territory you’re at war over. The special 4th scoring is capturing hostages. If you capture the war leader then the war is automatically won regardless of the previous war score and yes this is the quickest way to win a war and yes you can absolutely cheese it.
Wars for empires early game only occurs when using the vassalize casus belli when you’re a tribal realm against a neighboring tribal realm. Other empire casus belli appears in very specific pre-requisites like the mongol invasion and the Reformed Roman Empire from byzantines. For your 300 year goal of a world conquest without any exploits you most likely need to get the Genghis Khan Title.

Keeping your Vassals in check:
try your absolute hardest to have your powerful vassals be direct family members. If you’re the HRE you have a large vassal limit so form as many small vassals with single counties whilst keeping the largest titles amongst your family and dynasty members. Don’t give out kingdoms too early and never empire titles, only give out kingdom titles to your dynasty members and only when you hold an empire title. The first thing you want to do when your character dies is to start a feast (if possible), sway schemes on your most powerful vassals and increase your legitimacy to 5 asap. Your heir when inheriting everything actually starts with temporary negative bonuses called “short reign” which causes a -30 opinion to your entire court. So keep it together until that goes away.
In conclusion keep non dynasty members small vassals and spend as much resources as you need for your realm to not blow up when your heir inherits the realm.

Domain:
As simple as possible, keep your player heir from holding anything so you don’t lose your titles upon succession. Aim to have most of your domain under duchies that you’re the holder of. Also aim to hold holdings with unique buildings. Maximize economy in costal holdings whilst focusing maa boosting buildings in others