> Things like the compositor crashing
I had many crashes, daily, and thought it was due to KDE but then they happened with other WMs too. Then I thought of trying to run the CPU at a lower frequency - lo and behold, no more crashes. Sucks that I can't use my laptop to its fullest power but I'm too tired to deal with the constant crashes at max CPU.
> startup scripts not running in a reproducible way
Never had this issue - all the startup scripts executed as expected.
> weird behavior around multiple monitors
The only weird behavior happened when my DE crashed where panels moved to wrong desktops (I have a three-monitor setup). Once the crashes stopped, haven't had this issue and it remembers the panel settings but on rare occasions I have seen the refresh rate of one of the monitor change from 60 to 30Hz.
> dock sticking to the wrong edges
This never happened to me but see comment above regarding panel moving to wrong desktop (even then edge orientation was retained).
> every problem I've googled for (sddm most recently)
I have suffered from this SDDM issue too - eventually swithed to lightdm and everything works.
> When will plasma be livable in general?
I think it is livable already but thats what it is - just livable and I'm not sure if I would call it highly polished / rock solid - I still find rough edges E.g. adding each widget to panels increases CPU usage by 1% or so even though I'm not adding any CPU intensive widgets; moving icons on the panel is weirdly jumpy and some icons refuse to move; I have to install plugins and weird workarounds to enable panel transparency etc. I have seen panel woes mentioned multiple times in this thread and elsewhere; I think many people would greatly appreciate a feature to export/import panel config (even the entire desktop config) so that even if your panels etc mess up (ideally not but lets be real), you can just restore from the last known good setup. This would make such a big difference to the user experience.
The cleanest and most stable of the environments I have seen was Gnome 2.12 (I think that was the version with Ubuntu Dapper Drake). Intuitive, good balance between features and cleanliness and generally got out of the way. Since Gnome 3 and its radical changes came out, I have been searching for a DE that sticks to the conventional design (no going into rabbit hole of tiling WMs for me) and errs on the side of more features/flexibility. I did dabble with Cinnamon/XFCE/Pop desktop but I always felt that there wasn't sufficient flexibility/features (direct or via extensions). I think Plasma, with all its issues, caters the best to the aforementioned needs. So KDE it is for me, until something better comes along (which given the current ecosystem, is unlikely).