After some searching I can see one option I have is to force Wayland to behave like X11 by setting scaling to 100% and setting the environment variable QT_FONT_DPI to the actual DPI of my display.
This can be done for a specific user by creating /home/tumb1/.config/plasma-workspace/env/hidpi.sh
% echo 'export QT_FONT_DPI=162' > /home/tumb1/.config/plasma-workspace/env/hidpi.sh
% cat /home/tumb1/.config/plasma-workspace/env/hidpi.sh
export QT_FONT_DPI=162
The only item this doesn’t seem to work on is window-titles, but they can be customised under Settings → Text and Fonts to be something large like 16 pt - which then looks OK.