I know there is a lot of stuff on high to set DPI scaling for SDDM correctly, but I am currently having kind of an opposite problem:

I have a laptop with a very high resolution display using Tumbleweed with KDE and (some time ago) fiddled with the DPI/scaling/resolution settings and whatnot:
1. Using a different screen resolution than the default. This worked best to my experience (SDDM was tiny, but I only use it to type in my password, so that's okay) with some minor issues - the laptop resolution sometimes reverting to the default when I plug in a second monitor, for instance. The problem was that this stopped working at some point (I was not able to set the desired resolution for some reason) but I think this issue is fixed.
2. Using kwin's screen scaling feature. This also works but has the problem that a) windows on other monitors are scaled as well and b) I had issues with virtualbox
3. Using custom font DPI settings and icon sizes. Works as well without the virtualbox issue but has the same problem as 2 plus it looks ugly
4. Editing /etc/sddm.conf and setting an SDDM DPI value. This works with the same problems as 2 and 3

My goal right now is to revert everything to the default settings, i.e. super small everything on my laptop, and then using method 1 again since this worked best for me. The problem I face is that if I revert the steps listed above, I suddenly get a very huge SDDM login screen and a ridiculously scaled plasma desktop (konsole does not fit on screen). Heck, SDDM gets SMALLER, if I set the DPI value in /etc/sddm.conf to 192!

My guess is that I have somewhere overridden a default scaling/DPI setting that I'm missing out right now. Are there any other locations that I can look at to get an idea why everything is so large on my laptop?

Oh and on a side note: why does the SDDM DPI setting influence how kwin_x11 draws the window decorations? This seems odd to me...