I've been doing something very similar under Ubuntu for the last year or two (I manually launch a script that resembles your last xrandr snippet whenever I dock

I've been doing something very similar under Ubuntu for the last year or two (I manually launch a script that resembles your last xrandr snippet whenever I dock my hidpi ThinkPad with my 1440p secondary display). It tends to have varying degrees of success depending on updates that come out. For one, I have to repeat some of the commands in my script because otherwise I don't end up with the same end result every time. After it's done, for some reason the cursor arrow flickers when it's on the primary display. Sometimes, if I'm unlucky, UI window shadows will draw in the wrong place, and when I'm REALLY unlucky the script crashes X entirely. Right now I'm hoping that Wayland (or, if I have to, Mir) end up built in a way that makes this kind of configuration stable, intuitive, and sane.