@Australe
My suggestion to you is to get rid of all the individual buttons on the left and replace with a Gallery (let's call it galDepartments). Then add one button to the gallery.
Set the Items property of the Gallery to: ["All", "Administration", "IT", ..etc..]
Set the Text property of your label to: ThisItem.Value
Then set the Items property of your right gallery to:
Filter(
Office365Users.SearchUserV2({top:500}).value,
accountEnabled &&
((galDepartments.Selected.Value = "All") ||
department = galDepartments.Selected.Value )
)
I hope this is helpful for you.
EDIT: Forgot the accountEnabled...it is corrected in the above formula.