Perhaps I am terrible at this game, but early bog trolls sieges (3 in one year) are devastating.
Instead of weakening them, though, perhaps we could reuse an idea I remember from another mod some time ago - defense golems you could buy at the embark screen (or from dwarf merchants).
I think they were set up as pet animals, basically, but when they died they broke into rock blocks, and didn't reproduce. Essentially super war dogs without the need for training, but more expensive and lacking reproduction or ability to be butchered. This gave forts starting off in extremely brutal areas a chance to get going, as the golems could potentially hold off initial undead / savage animals / thieves.
As it stands now, early defenses are limited to stone fall traps and war dogs, both of which seem completely useless. Perhaps slightly buffing one or both of those would help?
Perhaps golem creation could also be worked into the tech tree, with more advanced golems being researched, and only basic ones available at the embark or from merchants. They could also have different qualities, so some would not be strict upgrades over others. Maybe obsidian has more damage potential with blades on it, granite is tougher? Perhaps a difficult to build high tech one with some kind of range attack?
Golems wouldn't replace your militia or other defense options as you can't order them, they can't train, and can't use gear. Care would have to be taken to balance them, but it's an idea. Having one bog troll get through my traps only to butcher an entire nephilim caravan plus guards, and then all thirty dwarves I conscripted into the military and set to kill him made me really wish for some early defense with a kick to it. Granted, they were largely untrained and unarmed dwarves, but still.....I'm still watching them go at it as I write this after conscripting the entirety of my fort into the militia, and the troll is healing as fast as he is hurt. He now has half a dozen scars, and basically every bone in him is broken, but it doesn't seem to phase him at all. I'm not sure how he can swing a club with two broken arms and two broken legs, to say nothing of the other injuries.
If caravans are ever fixed to bring their full compliment and wagons again, this would be somewhat less of an issue as they would have more guards and animals, as well as bringing much needed materials to an early fort. Still, bog trolls are complete death machines and can't really be handled by a militia unless they have some training and decent gear, neither of which a fort has to begin with.
Another way to mitigate this could be to limit the frequency and amount of migrants. I don't know if it's DF2010 or this mod but I seem to get boatloads of the idiots showing up very rapidly, which naturally brings the bigger threats along. I don't know if migrants can really be controlled, as I don't remember a mod ever doing it. This is probably the single biggest issue - without constant migrant waves, sieges wouldn't show up until later, giving more time to prepare traps, forge weapons, and train militia. This would also make individual dwarves more important, as well as medicine, as replacing dwarves would be more difficult. I really like the idea of caring about dwarf wellbeing. Of course, if forts grow more slowly they would have a greater need for early protection, too.
Perhaps have another race that attacks early, but in smaller numbers or smaller individuals, and move bog trolls up the threat timeline a bit? Perhaps a race that doesn't use items, and relies on natural weapons/defenses, so they could provide a threat without giving the fort any resources upon defeat.
Anyway, some ideas. And tears.
TLDR Bog trolls beat my ass, boohoo.