How are you duders finding the Realm Divide mechanic? It's essentially the Mongol/Timurid (or Roman Civil War) late game face rape in Shogun 2. Gaining "legendary" clan status and/or launching a bid for the shogunate by sieging Kyoto is more or less diplomatic suicide: you get a diplomacy penalty which eventually snowballs into a huge irreversible negative score. It makes retaining your allies impossible, and the AI stops fighting to begin a concerted effort to just crush you.  
 
I understand why it was implemented because without it you'd just ride on the momentum and smash everyone's faces in, especially since some castles are so close to each other that capturing two within a single stack's turn is pretty common.  
 
But Oda Nobunaga certainly didn't pave the way to a united Japan alone; he had to exercise shrewd diplomacy and political maneuvering.  Suffice it to say that this mechanic proves vexing for those who've traditionally played Total War games this way. 

THIS MOD HERE tweaks the Realm Divide mechanic as follows:  

 Realm Divide Mod v1.0

This mod changes the realm divide event significantly. Instead of a low initial diplomacy hit which adds up every turn by -5 points until you get a -200 penalty, you get a moderate initial hit (around -55 on normal difficulty) which gets reduced every turn by -4 points. That way allies and factions you have good relations with may stay on your side and diplomacy doesn't become impossible after the realm divide event.

The mod also contains some minor diplomatic adjustments: financial gifts and giving away you children as hostages both have longer lasting diplomatic effects (reduction of the effects each turn is now -4 instead of -5). 

 

I haven't played on anything above hard, but I can tell you from experience that if you play the game strategically on normal or hard, the game doesn't really even get interesting until the realm divide.  If you're willing to spend to do it , you can diplomatically avoid a multifront war.    Only once the Shogun steps in does that become impossible.  Though I'll admit I had a few rough years just before the divide when my geographically largest ally unexpectedly betrayed me in my lowest match.   
 
The problem with the mod's solution is that it's really easy to be on very friendly terms with most of Japan just through sustained trade, even without alliances.  It seems like they wouldn't attack you until way too late.

My inclination is always to push in one direction or another, so by the time realm divide pops up I've only got enemies coming in from a single side of the map.

" My inclination is always to push in one direction or another, so by the time realm divide pops up I've only got enemies coming in from a single side of the map. "

Its kind of a strategic bummer to have something like that chime in like clockwork. I do support any kind of variation on that aspect. I really honestly wouldn't mind from a gameplay perspective if they built campaigns around imagined maps (the modders I mean, or the devs, really.) I feel like strat devs in general put too much stock in how much people care about historical accuracy and not enough stock in what they can do with their gameplay. Civ 5 is another example, where you have like 6 unique civs you could make with the history of a place like Rome, China, England, etc, but instead you're forced to find some weird obscure shit from Polynesia. I can't think of a strat game where it wouldn't get better mechanically as soon as the devs stopped giving a damn about representing history.

They can keep the historical element, I enjoy it plenty myself, I just don't think they need to limit themselves to real world history. Cus you get a game like this, that is fun, yeah, but I played the map. I played Japan. I kinda wanna play a different map, you know? Land specifically designed with the game's mechanics in mind. Its not really aimed specifically at this game, this game just happened to remind me of a general beef I have with these sort of games sometimes.

It would be kind of too easy with that mod.
Realm Divide at least provides a challange as soon as you start gaining momentum, keeping the game interesting.
And if your allies are on good enough terms with you, they will stick by your side even with RD.