Now I found a solution that works for all the apps I tried so far:
add export GDK_DPI_SCALE=0.5 to /usr/lib/win-kex/xstartup
activate "Settings Manager" -> Appearance -> Fonts -> DPI -> "Custom DPI Setting" (leaving the proposed 192 next to the checkbox)
restart XFCE desktop session, and remove all the env QT_SCALE_FACTOR=2 prefixes I've previously applied to various QT-app shortcuts to prevent double scaling.