A fresh install on the machine mentioned results in the touchpad either being completely unresponsive or very spotty in Plasma.  This touchpad has worked under previous MGA versions, so I looked around for configuration options, but enabling the touchpad in System Settings and enabling tap-to-click had no effect.

I was about to conclude that the touchpad had become damaged (this machine hasn't been used in a year or more), but on a whim I logged into GNOME.  Initially, the bahavior was the same, but within minutes the touchpad was functioning normally.  I logged out and back into Plasma, and the touchpad was now fine.

KDE used to have a kcmtouchpadrc file that had all sorts of configuration stuff in it.  The nearest I can find in Plasma is touchpadrc, which has about 4 lines none of which pertain to configuration.  Whatever's going on, GNOME appears to be able to autoconfigure the touchpad, Plasma can't, but once GNOME has, Plasma can piggyback on GNOME's work.

The only out-of-the ordinary thing is that the Dell has a mouse button embedded in the keyboard as well.  That works very well.

The spotty or unresponsive touchpad behavior also occurs in the installer and sddm, if that helps.