Hi guys,

To this date I never managed to get proper multi monitor working. It has improved, but situation is a bit more complex now: I have a main 1080p monitor, and a secondary 4K TV to which the PC is permanently connected. The PC uses an Nvidia 1070 GPU with the latest privative drivers. I have all the following issues:
-Even when I set up the monitor as the main screen in Display Settings, and KDE aknowledges this by placing the menu bar in the monitor, the login screen goes to the TV. The monitor shows a blank screen until the password is introduced. Additionally, many new windows decide to open on the TV, not the monitor.
-Fonts look weirdly rendered, both in the TV and the monitor. This is because the Force Fonts is ticked, set to the DPI of 80. If I untick that, the fonts start rendering so small I'm unable to read them even when close to the screen.
-Scale Display on the Display Settings should help improve the situation...if it weren't a generic option that seems to apply to all displays at once, regarding their dimensions. I'd like to have separate scalings for different screens, just like what you can easily set on Windows.

Is there a way to fix these issues?

What version of KDE/Plasma/Distro do you have?
Plasma 5.11 added some improvements around multiple monitors and DPI settings, including being able to specify separate dpi per-screen (the release announcement mentioned this wrt Wayland support, but perusing the change logs it looks like at least some of the work done also applies to X).

It also sounds like some of the problems lie with the way that X has the monitors set up outside of any user session. You might go delving into the X config to see if there is something you can fiddle with there to swap which display is treated as the primary when you aren't logged in.