I think Garrett should activate atkspd more often because of how he's Ult and battle skill works.
A video is provided
The long video have these notes.
29 ults in total
5 ults already had 50% atkspd activated, doesn't count, since the artifact doesn't allow triggering again while it's activated.
24 ults without 50% atkspd activated.
only 1 ult activated the atkspd (the first hit, keep that in mind)
23 ults didn't activate 50% atkspd. (115 hit in total).
Either Silverstar is not 20% or Garett's ult's only 1st attack can activate atkspd.
Not sure about his battle skill, but can you test its 2nd and 3rd hit.
Thank you in advance.