Having a similar experience. Playing as a vassal to Raganvad, Sturgia. The raiders and caravans in the main quest always spawned on the other side of the map so I wasn't able to stop them about half of the time. I have almost exclusively played with all campaign options on realistic since release. Just had recruitment a bit easier in the first try because I was still learning. With this quest I decided to set the map speed to easiest just to have a chance to fight these spawns. We had Battania and the Western Empire down to 1/3 when the constant war with the Empire started. Was able to take down the Western Empire completely, only for a notification to show that all their clans have transferred to the other empires.
The reason I was still doing well and able to take out the Northern Empire was, imo, because I was given four towns close to each other (Varcheg, Seonon, Car Banseth and Dunglanys). But further expansion soon became impossible. When we almost had the Northern Empire on its knees Raganvad decided to go Robert Baratheon and take huge armies for random walks. Or maybe hunting. Except he didn't have the decency to die. He's still doing it. "Oh, I'll go to our settlement Diathma. What is that? Saneopa is besieged by the Southern Empire? Meh, it'll be fine. Actually, let's go to Omor. I feel like hunting some moose." Also, as soon as Battania would declare war they would set all villages on fire within minutes. It made me download one of the most successful mods, Improved Garrisons, just to have the towns survive.
Even when I had 62 prisoners against their (Empire) 11 prisoners, their power level would be higher.
And STILL I kept on going, until finally I reached a pivot point, many years later:
Then the Khuzaits showed up... They hadn't been at war more than twice this whole playthrough and suddenly I was facing an army of 2000 with more than a quarter of those troops being horse archers. I had had enough of being messed with and this is where I went mod crazy. First, Better Combat for extra health, health regen and the cut through cheat. I finished that fight with about a thousand arrows in my body, fighting the last 200 troops solo. Up to this point I had been playing full honorable, didn't break the law even once or raided any village but I was going to take the head of the guy that brought the most horse archers to that army. Odofled, who turned out to be a lady, was decapitated.
Got all the "Better" mods. Great series. It's nice to give companions and lords a 1% health increase per level so they become meaningful allies/opponents without going full Dynasty Warriors. Saw that True Relations is updated again, so got that too. Tried Distinguished Service, which is amazing. Also using Realistic Battle Mod (AI only), which is good but a bit too focused on the most elite side winning imo. More for great fights than reaching world domination. Don't Stop Me Now to give horse charges a little more oompf but also self-inflicting damage. Carrier is nice for night time torches and some extra flavor with cheering and responsive troops and a banner morale system.
And still I wasn't making much progress. I had slaughtered thousands of troops single-handedly by this stage. Running from Lycaron to Syronea back to Dunglanys, round and round the map I went. Sometimes managing to fight the armies before they finished their siege, sometimes not.
All the mods I'd been running did not have any dependency. No MCM or Butterlib or whatever. I did get Harmony because that looked very stable while the others showed that they would (inevitably) occasionally break when the game was updated. So I did have a look at Diplomacy but didn't like the list of required mods to run it. Finally, yesterday, I found Keep Own Fief. Now, whenever a castle or settlement has been taken, I get first pick.
So now I'm literally cheating my butt off and it's still going to take me a while to get there. The other kingdoms aren't really fighting each other anymore. I've almost taken all the towns of former Vlandia. Stuffing the outer garrisons with 500 troops and training them to level 5 until the walls are level 3. I've taken the Vlandian king's head in the process, just out of frustration. When his son decided to attack Pravend I took his head as well. After close to 4000 days in-game playing almost exclusively honorable, I was now level 1 dishonorable. Luckily True Relations will fix this a lot quicker than the base game would have. Although I don't know why I should even care...
Battania is around 400-500 power level with 2 clans but still manage to take settlements and castles occasionally when they can afford several (!) mercenary groups (how?). Why don't they cease to exist like the Empire at some point? Do I need to take their heads as well?
Vlandia, Southern Empire and Khuzait are all around 5000-6000 power at the moment while Aserai is around 13k. We are at 22k. Raganvad is still alive, in his 70s now. The useless bastard hasn't helped more than twice in the last two decades. He has started to give away his fiefs so I'm hopeful he'll kick the bucket soon.
I understand the campaign end game balance is probably last on the list in development. It's probably better to play the sandbox for now, at least if you plan to play on Bannerlord difficulty and not cheat. Right now, the campaign looks like you're not supposed to win.