The alternative to borderless mode is to go into properties-compatibility for stellaris.exe and tick the Disable fullScreen Optimisations box.
Okay, so in fullScreen you can only use 3840x2160., is that right? Is this because other resolutions do not show in the launcher? Or they show but don.t stick in some way?
If you do use that resolution, and up the GUI scaling to say 1.2 , is that more usable for you?
You should go into properties-compatibility in stellaris.exe and tick the Disable fullScreen Optimizations box, that might help. I hope so!