Colzet triggers "At the start of the End phase..." and R5 Astromech triggers "During the End phase..."
1. Colzet would trigger first flipping a random damage card and then R5 would trigger to flip down a ship damage card regardless of initiative order because they aren't at the same time.
2. Or is "During the End phase..." too vague and the start of the End of phase is just as valid as any other time during the entire End phase for R5 to trigger. This means it comes down to initiative order as to how the two abilities trigger.
#1 seems correct to me as "during the phase" and at "the start of the phase" feel like unique triggering timings.