Use the Freeze Frame Timeline action to create an effect where the video pauses for a short time. This is not for playback control during editing. This action was added in Shotcut version 25.05.
To use this move the playhead (also known as the “cursor” or rather grandiloquent “current time indicator”) to the time position where you want to hold. Then, right-click the clip and choose Freeze Frame. This creates a one second clip with only that frame. To change its duration, either turn on Ripple mode on the timeline toolbar or press and hold Shift. Now, drag the right edge of the clip to adjust it.
Feel free to make your own keyboard shortcut for this action in Help > Actions and Shortcuts.