Defer will only pause long enough for other processing around it to finish (in this case, there is nothing else to do…). Instead of the defer node, you will need a delay node between the time you set the servo angle & the time you take the reading. The servo can take around 0.2 seconds to reach its target destination.
If you find the servo jerking back & forth instead of moving to a new position, that is a different problem that needs another solution.