G28 z0 first plunges down

A little one. G28 Z0 will go to G28’s Z position through Z0, assuming you’re in absolute mode (G90). That means it will go to your work Z zero before heading to your G28 Z position.
You could use relative positioning. G91 G28 X0Y0Z10 will move up 10mm in Z, then move XY&Z to G28. Be careful bouncing around from G90 to G91, though.