I agree, your DS2 is probably close to outgrowing the summit, and is still too young for a booster. Also, the base of the summit does not make a backless booster. Could you be thinking of the Safety First Intera
? If it is the intera, you can remove the backless booster from the bottom to give you a little more room with DS2 using it as a harnessed seat. Your DS1 should be in either a high back booster or a harnessed seat, low backs are for children who no longer fit in high backed boosters. Your DD would still be 500 times safer RF.
If I were you, I would look for a sale on the Sunshine Kids Radian
for your DS2 (keep looking for sales), and at the very least a highback turbobooster
for your DS1. You DD can continue to use the scenera until DS2 is old enough for a booster or she outgrows it. I would turn her around and keep her RF as long as possible!
If I could not afford car seats to fit my child, I would not be buying a new car. I currently drive a 1993 Buick LeSabre, and hubby drives a 1998 Chevy Blazer. My almost 4 yr old and my 5 yr old are both in harnessed car seats as they lack the maturity to sit in a booster seat correctly 100% of the time. I had top tethers installed in my car, and hubby's car allows tethering in the outboard seats.
There are cheaper options that would work in the rondo, since you have more space, but since you are looking for seats that may work in the soul, that is what I would get.