First I would like to point out that when you shorten a track, then the shortened part will not be deleted, the notes are still there, waiting for you to re-lengthen your track.
Secondly in the Pyramid-land you have to be best friends with the “zoom” level of tracks. In this instance you could use the “zoom” level to have one bar correspond to one step in STEP mode. This would allow you to empty entire bars and also move them around while holding REC + < / >. Once done, just adjust the “zoom” level back your preference.
Tip: You can change track parameters in any mode. While in STEP mode, press and hold TRACK+ZOOM and adjust the value with the main encoder OR press the corresponding pad for super mad skills.
Hope this helps