This question I posted on the Qt forum has been identified as a potential bug:

https://forum.qt.io/topic/116573/how-to-cater-for-windows-user-text-size-in-pyqt5

In brief:

On Windows 10, Qt is adapting its display size correctly when the user has a custom DPI setting set (via Windows Settings -> System -> Display)

However it only partially adapts the font size correctly when the user has set a custom text size (via Windows Settings -> Ease of Access -> "Make text bigger")

This is resulting in Qt dialogs that are difficult to read for users with sight issues or large text requirements, because while some elements are scaled (e.g. status bar, menu bar), the elements in the main body of the window are not scaled. The text remains small and difficult for them to read.