I’m a CPST who reviews car seats to make your search easier. This video covers how to install the Graco slimfit3 LX and Graco True3fit LX in Booster mode.
I honestly dont get this seat. It has the same max height for the graco nautilus yet my graco nautilus headrest goes way above this seat. My son still has at least 3 positions on nautilus yet this one would only fit him very temporarily. Something is off. I got this seat for my 5 y.o so not a big deal in those terms but would have been useful to get one for my boys as well since they are slim in design. Ended up getting one of them a peg perego flex 120 and keeping the other in the nautilus.
This seat is an all in one. The nautilus is a combination seat (often referred to as a harness booster). It is common with convertible seats that they do not have as high of a top strap/booster height as a combination seat or dedicated booster. The all in one is designed to fit tiny babies all the way to booster age whereas the combination seat is designed to fit larger children only. You will be hard pressed to find a convertible/all in one that lasts as long as a combination seat in forward facing harness or booster stages. That is why most CPSTs do not like the “last seat you’re child will ever need” marketing on some all in ones. There’s no way that’s possible.
The gray strap item you referred to as the buckle. You moved it up in front of the storage space but I don’t see where you did anything besides leave it hanging. I’m trying to figure out what to do with this part. Thanks
There is a gray door on the back of the car seat that you open and put the chest clip and buckles into to get out of the way for booster mode. Then you close the door and remove the crotch buckle
