I definitely do agree that dwarves are brained too easily. This is particular problematic with the dwarven injustice system, where a single dwarf can murder dozens of dwarves in a single punch each time, which I think is far too lethal.
It also means that the healthcare system isn't used frequently, as injuries are all too often binary. Either a dwarf is perfectly health, or they get brained and instantly killed. There isn't much in between.
Reducing the chances of being brained would also make things more interesting, as you would have a dozen badly injured dwarves slowly healing up in the hospital or too injured to work or fight again, rather than simply having a gap in your roster which may trigger a new migrant wave, instantly replacing your losses with a new dwarf.
Having to take care of the injured veteran warriors would be an interesting addition to a fortress.
As an aside, is there any good way to modify the raws so brains are more sturdy?
[NO_THOUGHT_CENTER_FOR_MOVEMENT] is something I tried. Dwarves will still bleed out, die from being chopped up, and will die from infections, they simply cannot be brained even if they get knocked unconscious and have an entire goblin siege smashing the poor dwarf's head into paste. While this tag certainly makes dwarves immune from being brained, I think it makes them too tough.