Question How long will DS fit in a Radian?

I have a 2001 Subaru Outback with 2 Radian 65s and one Graco ComfortSport. Using Radians is the only way I could figure out to get 3 seats across the back. I have 2 sons (Radian 65 & ComfortSport) and babysit a 3-yo girl (Radian 65).

DS1 is 4y1mo, 40 lbs, and 40 inches. He's in a Radian 65 with the straps in the top slot. There are 3 inches between the top 2 slots, and he looks like he has about 2 or 2.5 more inches before he outgrows it by height.

DS2 is 21 months and in the ComfortSport. He won't last long in there (by height), so I need to figure out a new solution.

Sooooo..... here are my questions:
How much longer will DS1 fit in the Radian 65?

Do I get DS1 a different seat with 5-pt harness that he'll fit in longer by height? (If so, what seat is no wider than a ComfortSport and preferably would convert to a booster?)

Or do I keep DS1 in the Radian and get another Radian for DS2?

Is there any difference in slot height in the Radian 65 and Radian 80?

We're going to need a new car (probably a minivan or crossover) in the next year or two, but in the meantime I need to squeeze 3 seats in the back of my outback.

I've heard that children, in general, grow about an inch of torso height a year. The Radian will fit most to about 6 yrs old. All of the Radians have the exact same shell so the top slots are the same for all. I'm not familiar with your vehicle so I'm not sure I can offer any seat advise.

The Radian allows you to use it with the child's shoulders over the top slots. It is outgrown by height at 53" tall* or when the top of the ear goes over the top of the shell, not when the shoulder goes over the top slot. So he has 13 inches of growing room.

* Current models are 53 inches. Earlier models were 51 inches or 49 inches. I don't know if the change was retroactive. You should read your manual, the stickers on your carseat, and contact SKJP if you have a 49 or 51 model and want to use it past the stickered limit.

I don't know if the outback's backseat is bigger or smaller than the impreza's back seat but I was able to fit 2 radians rear facing outboard positions and a gn center in my impreza.

Oh, awesome! I read the Radian owner's manual but didn't originally catch the one sentence where they say you can use it even if the kiddo's shoulders are above the top slots. That info made my day. Thank you so much! We're going to keep DS1 in his Radian and get another radian for DS2.

Such wise mamas (and papas, probably) around here... Thanks again!