do you know if using the save button in ironman kills achievements too? Its the only thing i can think of since otherwise ppl would just save, backup the save, resign to title screen and paste overwrite the save that happens when you do that - instead of task manager
Using the Save button doesn't break achievements, I use it all the time. It just overwrites the same file that the Autosave, Quicksave, and Exit/Save-And-Exit buttons do.
As for manipulating saves vs using the task manager, a couple of hot-keys is much faster to me then bothering to manipulate the save files. I actually really enjoy Ironman and most of the restrictions. I'd probably use it even if I was playing with a mod or rule that disabled achievements. Only keeping one file and having very limited abilities to undo or reroll events provides a happy balance for me.
I lost interest in CK2 when it was still version 1.x because editing the save was too easy compared to the short-term frustration of things going badly - even though in the medium and long term it killed any challenge to the game and the fun I was having with it. I like that time-travel isn't an option (a term a friend and I liked to use for jumping back to much earlier saves in Civ). It really does make the choices meaningful. (And if I really wanted to branch a game and take two different forks I still could, but I've never been interested enough to actually do it.)
At the same time, I also don't find any value in letting a misclick wreck hell on a save that I have been playing for three months. Like surrendering when I was trying to enforce demands, a window popping up as I try to click something else, or one of my cats stepping on the key to toggle off raiding for the army that is currently funding my war. (Why is that even a single button? Given you can't reactivate it during a war or if you are out of your territory, that seems like a better option for a combination.) I will occasionally use it to force a re-roll for an event, but that's generally because I have some story I'm working on and the event wrecked it out of the blue in a way that I don't find interesting, even if that event is one that is a bonus for my character. I know it's a completely subjective line, but since I'm playing to an audience of one, I don't really care.
Killing the process tree and restarting is also just enough of a pain that I don't want to do it often. (Though admittedly it is much less of a pain since I switched to an SSD. I think it made an even bigger difference in the start-up time for CK2 than it did for booting Windows. It was kind of impressive.)
Finally, I have cloud saves turned on to keep a connection outage from breaking achievements. I haven't tested but I think Steam will notice if you switch or change the file while offline (which is why losing connection and having it autosave was breaking achievements). I'm not sure about that though since I haven't really tested it.