Firstly, the ‘opacity’ setting seems to be stuck within Pressure > Pen settings:
It does work as expected and I can still change the min and max values (so I can essentially disable it by setting both to 100%) but I can’t uncheck the checkbox.
Second thing is that I have two tablets attached to my computer - a Cintiq and a Huion tablet. The Huion tablet is the one most recently installed and now seems to be the only one recognised by Pro Motion. Even if I disconnect the Huion tablet’s USB cable, it’s still listed under Preferences > Tablet/Pen and no pressure sensitivity is recognised from Cintiq input. I am able to use the Cintiq normally in other software, even with the Huion attached.