It should be possible to activate/press/click tool buttons (QToolButton) and command buttons (QPushButton) with the Enter and Return keys. QToolButtons can only be activated with Space and for QPushButtons it depends on the default and auto-default property.
For tool buttons a solution exist: rKLEOPATRAccb6f078db56: Add and use a custom tool button
For command buttons a possible (hackish) solution is to set the auto-default property when a button receives the keyboard focus. Subclassing QPushButton isn't a viable solution because Qt and KF5 create loads of QPushButtons out of our control.