So is it even possible right now to play a fortress with sieges and everything without an inevitable total collapse and without having to micromanage the absolute shit out of stress?
Yes. I have run a successful 30-year fortress in a haunted tundra with regular sieges, ambushes, and undead, so it can be done.
First, a note on stress: while it is possible to overwhelm your dwarves with stress, in my experience most "stress fatalities" are actually caused by personality changes either increasing the amount of stress or reducing the ability to handle it. A few "personality altering events" are much too common, and typical "good thought" tactics do nothing to combat the resultant personality changes.
Option A: Dwarves are expendable. Thanks to conquering and expulsion, you can easily retire overstressed dwarves off-site, to be replaced by unstressed migrants. At one point I probably replaced the majority of my 30-dwarf population in a single year. While losing peasants and potashmakers isn't particularly painful, exiling a legendary armorsmith really stings.
Option B: Avoid personality changes. You can avoid most of the common "personality altering events" by altering your playstyle. Things like not sending cave-adapted dwarves outside, avoiding inclement weather, assigning a dedicated "corpse cleanup" squad, and making sure you have clothing and goblets will significantly slow down the rate of "stress fatalities". My expedition leader has been with the fortress since it's founding (thanks to a good starting personality, protection from personality altering events, and some luck).
Option C: Cheating. You can mod dwarves to have higher stress-resistance, or just remove their stress with DFHack.
A few recommendations:
* Some personality altering events can be almost entirely avoided, like embarking in a desert to avoid rain, or building an above-ground fortress to avoid cave adaption.
* Stress is cumulative, but the chance of a personality change is not. Instead of spreading the stress widely (and risking many personality changes), assign problem tasks like corpse cleanup to a small, expendable squad.
* Some traits can mitigate certain bad thoughts and avoid any associated personality changes. A dwarf who "only grumbles mildly at inclement weather" is immune to rain and snow personality changes; a dwarf who "doesn't really care about anything anymore" can haul corpses without risk.
* Libraries can provide a barrage of happy thoughts (particularly if you have reserved books you can "release" when needed).
* Using the military equipment system to assign cloaks to stressed dwarves can grant several happy thoughts (one per quality-level). Enabling the mining or woodcutting labor will cause that dwarf to regularly re-acquire the cloaks for additional happiness boosts.