900 hours? That explains it lol--I most likely just lost a lot more often than you did
That, and I'm prone to turtling and waiting for things to happen in such away that I can best take advantage of them.
I wouldn't play on Ironman simply because (I'm on PS4) I can't trust the integrity of saved games when they were consistently corrupted whenever overwritten (so I had to make a new file each time to save and delete older ones for room). That, and I'm not ready for the realistic consequences of failing dating in Calradia lol. I reloaded after my first final date with Adalindis dey Folcun ending in failure and succeeded the second time and, to make myself feel less like a cheater, instituted a "three strikes rule" which I abandoned after my poor daughter/successor couldn't find a (younger) man without buying one lol. I was willing to maintain the realistic no-save scumming rule with failed sieges and repeated imprisonments lol, but I made marriage an exception (for now lol).
Some clan leaders are just that loyal. Is there a hidden "loyalty metric" being calculated or something? No clue lol, but I chalked it up as "some people are just really decent dudes who'll stand by their homies even if they're homeless and repeatedly imprisoned for it." Mad respects to King Ymir Gundaroving who remained loyal to Great King what's-his-name of not-Gundaroving who, despite being the son of the former ruler and my bestie after a while, remained faithful. Several Sturgians kept the faith like that lol so I do wonder if there's some special programming with some of the rulers or some hidden mechanic going around.
But, you don't really have to recruit everybody to unite the world. My approach was to recruit many vassal clans to overwhelm my opponents (which worked) but it probably isn't necessary. For reference, I believe I counted 30 or so clans by the time I unified Calradia (or about 3-4 times as many as a default country's clan count) so you're likely strong enough to let your vassals handle everything from here if you'd like to take a break "in-universe" lol. My breaking point was when I united 4/5ths of Calradia and, having suffered so many crashes related to sieges, felt my patience evaporated and wanted to call it a done game, but I chose to wait out the clock from Ostican (my new capital since my vassals wanted to give ma the Christmas present of northwestern Vlandia lol--I'm only half joking by the way) and, after seeing my vassals gradually mop everything up I decided to put down the neutral city states in Vlandia, finish off the Vlandian Kingdom (I let it be in the single castle between Jalmarys and Ortysia), and finish off the Western Empire (which had united nearly half of Calradia before them and us became the final two major factions) in Vostrum as my final battle before improvising a credits scene where I returned to Ostican while listening to appropriate RTK7 ending music and looked through the codex of familiar faces to see all the stuff that happened over 48 years lol.
Now, as for proper advice; I recommend handling the Nahasa Desert yourself and let your vassals deal with the remnants of Sturgia and Vlandia. You'll probably enjoy yourself more as I did semi-soloing Sturgia and I recommend pulling in friendly retinues only when it's time for sieges (and then disband them between sieges so they can recruit fresh troops and you can move faster on the map). An important trick to staffing castles and cities you might not know about is to repeatedly exit and reenter when while commanding an army; an important caveat is that once they're voted for somebody not called "the player" they're far more generous about it, so hanging out in the conquered fief until voting's done is recommended so you can let your vassals donate enough troops so the soldier count is ~100 before moving on to the next siege. However, retinues eventually run out of "spare soldiers" so ejecting them temporarily is a useful means of allowing them a chance recuperate ejecting a handful is a great way of scattering local enemies you otherwise can't pursue (not to mention the bolder among them will actively attempt to pin them down for you to finish off).
Another thing to notice is that, in the diplomacy screen, you can choose between "Aggressive, Neutral, and Defensive." In my experience, this most affects the likelihood they'll both invade the enemy country AND pursue their units. I recommend setting policies with all landless factions to "Aggressive" so they're basically screwed as much as possible but set ONLY the country you want your vassals to invade (I recommend Sturgia and the purple guys in Omor in your case) while having the ones you don't want them invading Neutral. This way, you can have some control over where they choose to invade since they typically invade the country you've set to Aggressive if the other landless factions aren't marked as "be aggressive to them" accordingly.
It's okay if a handful of clans have 6 to a dozen fiefs while most have 1-2. That's feudalism for you lol, and while there's the obvious risk of them leaving off with those territories I doubt it's likely to ever happen since I never happened to me in my 48 year playthrough and only a couple clans (namely fen Gruffendoc) defected between AIs (and fen Gruffenduc fulfilled the encyclopedia's implied conditions of "poor and cornered" at the time of flipping Epicrotea to Sturgia lol). The wealthier clans have a much easier time fielding large armies and gathering lots of Influence so they can unite Calradia for you while the little guys will typically follow along. I actually don't recommend donating your own fiefs to vassals at this point because they're just a drop in the bucket while your own private source of wealth is how you not only maintain veto power (especially if Serfdom's enacted) but also secure the throne (most successions favor the most powerful, so it helps to be the largest/strongest land owner to maintain the monarchy). Finally, there's diminishing returns with your vassals ability to vote so 4 guys that can spend 200 Influence plus 10 guys spending only 50 really isn't worse than 22 guys each spending 50 each with 6 others spending 100 or so. What I mean to say is--it's not so bad for power to concentrate into a handful of vassals provided you're still way stronger than them. You always have the option of voting to exile them (effectively stealing all their land in the process) in a pinch, and, in my testing, it's not much more expensive than voting to seize a person's fief and subject it to voting lol.
Finally... well, that's about it; where you're at, I'd recommend chilling out and letting your vassals take it from here while you play pick-up or finish off the little guys solo-ish so that you can have some decently balanced fight encounters at least. However, if you were earlier on, I'd also recommend using Trebuchets to destroy walls since I found that a very effective way to minimize allied casualties and make city/castle taking very easy (and I manually did most of them for the specific purpose of ensuring I only ever lost 2-4 guys per 1-200 enemy defenders lol) but crashing bid me to just automate it (even then, losing 30-50 guys against 1-200 isn't that terrible by the point you're at lol) except for the final siege (which I wanted to do manually to give it an Osaka Castle vibe lol).
Hopefully all this helps! I can't think of much else to say other then, while I did have gripes with the slow clean up at the end, I really did enjoy the unification phase and the pushes and pulls. My favorite part was where, expanding from the Nahasa Desert after toppling the Aserai Sultanate, I had to choose between maintaining my territories in the East (Vostrum to Odokh) or maintaining the territories in the West (Ortyisa to Jalmarys) since I realized while under pressure from the Vlandians, Western Empire, and Southern Empire (they were based in the Duzeg Steppe lol, Emperor Nemos of the Western Empire had managed to successfully unite all the Empire lands proper lol) made continued expansion in both sphere impossible. Ultimately, I forsook the east to focus on unifying Western Calradia since I figured it'd be the perfect launching pad to just push east for the rest of the game. And I succeeded--at the cost of all my conquered eastern lands and Husn Fulq while having to retake Lageta and nearby lands from Sturgia lol. And then I had the fun of dealing with Sturgia while I let my vassals keep the Western Empire distracted, and then pincered the Empire from "behind" via Tyal (a neutral city) and... well, I got bored once I realized I had basically won from then on lol.
