I had the same issue with my 14,0" WQHD (2560x1440) laptop screen and my external monitor. I wanted to scale up only the laptop's screen so that I can read the text on it, but then my external monitor got scaled too large.

After using it for months like that I solved this by simply setting the resolution on the laptop to 1920x1080, and scaling to 1. I felt really stupid after that I did not do this sooner.