Problem is with your QT library, why can't you update to newer version? On VFX reference platform this bug was reported 6 years ago O.o
Something is completly f***up; in Manjaro - KDE, 3840x2160, scaling to 150% works like a charm....except viewport, they are tiny, but UI INCLUDING fonts is scaled properly, see screenshot, no - i did not use scaling workaround, all fonts are clear and sharp (UI and settings). Could not replicate this on Windows. Also Designer is scaling properly except Menu fonts.
Seriously, there is not a single person with 4K monitor in your office? Your scaling problem was reported since HiDPi became new norm, I bet there is bug reported by Moses.
New materials in Substance Source are cool, but most of us would prefer software that we could see what we are doing, especially graphic software. Foundry Mari is using QT library and yet you can customize fonts and size as much as you like - killer feature to Painter coffin???