[dandanua]

You cannot imagine what will be in the spaceship at the speed exactly c, the "asymptotic" thinking doesn't work here, just like some number sequences or functions don't have limits at some points.

At the speed of 0.9999c the metronome will be ticking exactly the same for an internal observer. An observer from the Earth will notice time dilation, so that ticking cycle will be slower than "normal". If the speed of spaceship remains constant, then the times of back and forth cycles will appear the same from Earth. Simply because same time intervals are delayed by the same amount in this case.