Graco 4ever car seat

Hello! So my son is now 6 months old and 18.5-19 pounds and I think it's time to get him out of the infant car seat. We bought the Graco 4ever. I'm just a little clueless and I've scoured the manual and online reviews and such but can't figure out a couple of things. There are 3 recline positions in rear facing but they don't really specific which should be used when. I guess the first one would be newborn but which position should it be in for a 6 month old? He does frequently fall asleep in the car as well and of course I'm nervous about head slump. Also, how long do you use the infant insert for? Just until they are too tall for it? I'm assuming it's also to help fill in any gaps if the buckle does not come close enough in the closest position. Seems cozy. Haven't put my son in it yet but how long did you use it? And what about the shoulder strap cushions? Necessary? To a certain point? FTM here and trying to learn as much as I can about car seat safety and want to make sure I've got everything right. Thanks!