Re: The ONE thing I can not figure out!
Yeah, you're not going to have to buy only one seat. There isn't a seat that lasts from newborn to done with carseats and does each stage safely and age appropriately. Also kids generally don't pass the 5 step test in a vehicle until 10-12 years old.
There are definitely seats that will last you until you need a high back booster, and you can just purchase one then, which is a few years down the road.
I would also suggest the True Fit. It sounds like it would be a good option for you.