I still get my bash shell and unix experience and tooling easily as well so there is very little context switching when I jump into a linux container or remote host. There's tiny differences in some of the commands but largely a consistent experience.
I love linux, but the desktop is hard to get all the little things done right. On server, Linux just kicks ass, and all the tooling is built for it.
I have a Surface Pro 3 that works best at 175% scaling, and every day I connect it to a 27" 1080p monitor at 100% scaling. When dragging a window between the two, it will switch to the other scaling factor when it's halfway across. It works very well, so I'm not sure why it doesn't work for you. Maybe take another look at it?
I have also experienced the pain of trying to setup Ubuntu on my SP3, and eventually gave up and went back to Windows because it was such a nightmare. Which is weird, because I seem to remember that Ubuntu used to support non-integer scaling factors in the past.
The problem then becomes, if you don't have a 200dpi display, the scaling is noticeable.. Just like if you scale the display on a non-Retina monitor on a Mac.
You shouldn't really need to scale 2560x1440 @ 25" unless you have bad eyesight. Go up to a 27 or 30" display.
edit: I just installed Ubuntu 19.04 and it has GNOME 3.32 but there is no fractional scaling available in settings, in either Xorg or Wayland. The old experimental option can still be enabled but continues to make non-GTK3 apps blurry.
I didn't know if it was Linux, the TV or the Nuc. So I spent the next two hours trying to figure it out to no avail. About to give up, I then had what I thought was a dumb idea, hook it to the TV using an HDMI-DVI cable. It worked. the 1920x1080 desktop filled the screen as it should. The only thing I can think of is the HDMI port is only setup to look for TV video profiles. The DVI port expects a PC so it can figure out the signal and get the picture right.
It's a shame that they nailed this, but decided to remove menus, desktop icons, system tray icons, etc. GNOME could have been a great competitor to macOS and Windows, if they actually talked to real-world users. I wish that they had an UX steward like Sun Microsystems back in the days.
I do like things like window snapping, and miss it in MacOS, but the actual desktop experience is so much better in MacOS than pretty much anything else.
[1] The developers are doing great work, but it is a uphill battle without support from NVIDIA.
If I am not wrong gnome 3.32 also has the nvidia wayland backend
Messing with Xorg config files is something you do not have to do since Xorg was forked from XFree86, especially when it comes to monitors. Personally the only time i had to do that was to force 1:1 mapping for mouse (ie. disable mouse acceleration), but that is not monitor stuff.
I settled with i3 reacting to my connecting/disconnecting my laptop from my monitor, but even then the resolutions never looked correct.
I ended up selling my Lenovo Carbon x6 and bought a macbook pro. I was so bitter about the experience, but on the other hand I'm no longer worrying about things that should just work.
/rant
I also frequently connect my XFCE laptop (only 1920x1080 native) running a totally different distribution to a 4K TV.
This requires a visit to the Appearance menu (as above) every time I plug/unplug in order to adjust font sizes. (Or, I could just run the 4k at 1080p, but either way it needs an adjustment.) This doesn't actually bother me, since it's just one step and I could script it if I did it all the time.
As far as using your laptop screen as a second screen, I've never really cared for that as a way to work anyway because of the size disparity (my 4k monitors/tv's are all much, much bigger than my little 13" laptop), but you could play with the resolution on each -- but you're right; the DPI settings in XFCE are for the whole desktop (and all apps, including Firefox), not per screen.
I'm using Compiz, fwiw.
Works great no matter what monitor I plug it into using an X1 Carbon with the hidpi screen.
No problem with KDE Plasma, but when running a tiling WM without a DE...
1. Plugged the hdmi, everything looks so small. 2. Mate desktop had HiDPI support but only by a scaling factor of 2. Now everything looks very large. 3. I have to manually change the fonts dpi settings. Chrome and some other apps look good but the menus, file manager and firefox are still upscaled. 4. I manually change layout.css.devPixelsPerPx in about:config to make firefox look good.
Finally, the result looks very unpolished. The menus are still largely upscaled and some apps like vlc won't even scale at all. My laptop's screen can't be used as a secondary screen because everything is upscaled, not to mention that i have to manually undo all those steps to revert everything back to "normal" for my 15.6" laptop screen. I eventually wrote some scripts with dconf to automate the proccess but it's just not good.
Does anyone have experience with external monitors and linux desktops? How does xfce or kde handle this? I tried gnome that supports fractional scaling and it's way better than mate. Now the problem is that i only have 8GB of ram and gnome shell is known to be a ram hog.