The 1440p monitor issue seems like an especially bad bug. I run a 2160p monitor at a scaled HiDPI resolution (3008x1692) and never run into that. The discrete GPU idles at around 9W.
(Though also using a thunderbolt -> DisplayPort cable might be helping? Connecting over HDMI could exacerbate things).
I know you're not going to like it, but you can make it go away by switching to a slightly lower resolution. So the external monitor will be doing the scaling. It's slightly fuzzy, but the quietness is golden.
(Though also using a thunderbolt -> DisplayPort cable might be helping? Connecting over HDMI could exacerbate things).