As of November 2022 there's no proper/flawless support for "fractional" (125%, 150% etc) scaling of the GUI on the Linux desktop. Yes there is good support in KDE, but still some apps (e.g. darktable, steam) either ignore or wrongly position control elements on the screen. And there's no proper support for multiple displays with different dpi (as far as I know it's a limitation of X11).
If you have a laptop with small physical screen but higher pixel count (e.g. 13" 1920x1200), then i found the only working solution -- adjust fonts size and use some settings inside the applications to increase readability, e.g. set default zoom size to 150% in Firefox, set bigger fonts in the terminal emulator, set bigger fonts in the VSCode preferences etc, set 150% scaling in the Telegram app preferences... But this will not allow for different scaling for the external monitor.
If you plan to buy a laptop with small physical screen, look for either a 1280x800 screen or an integer multiplier of it, e.g. 2560x1600, it's what the Apple laptops have -- they do not support fractional scaling, but the display resolution is twice as large as a previously common one. By the way 16" macs with 4K displays have higher dpi than previous 2560x1600, so the texts are uncomfortably smaller and there's no solution to it at all.
If you plan to use a multiple display configuration, make sure that displays have the same or at least close DPI value.
