Chicco Keyfit harness help

Gizem

My LO is almost 5 mo old and seems a little squashed when in car seat (Chicco Keyfit). It was pretty obvious when we needed to move the straps to second level, but not sure when to move to the last one. I do not want to remove his head support pillow which will be the case when I go to the last level.

When did you move the harness and take off the head pillow in the car seat? How did you decide?