Thanks for the links, particularly the second one. It looks like something I may have experimented with before but I'll try again. It could be that my touchpad does not support ABS_MT_TOUCH_MAJOR (Dell XPS 2-in-1 2019).

My "issue" is described as expected behaviour in this documentation: https://wayland.freedesktop.org/libinput/doc/latest/palm-det... (the "A" event).