Didn't occur to me yesterday, but maybe you could look at different booster seats for the eldest two? For example the
Britax Adventure or
BeSafe izi Up -- those have no armrests and are probably more easy to buckle than the Rodi and Pallas.
I will see if I can find those In the store. the rodi doesn't HAVE arms EITHER (SORRY 2 Yr old playing on the caPS lock). They still might be smaller/not so wide though. But I could only do that for my oldest as my middle child, I could not trust him in just a booster. When we used harnesses seats before when he was still in the infant carrier, he would 'Houdini' himself out of the harness, no matter how tight it was. :-/ Things haven't changed much, one day, recently my husband put him in the Rodi in a hurry (oldest wasn't riding with us) a few minutes down the road, he is sitting in between the front seats. Also he is tall but skinny and just barely 14kg, so he technically can't ride in the boosters yet.
I went to a local baby store today and got really frustrated because the lady had her own selling agenda and didn't listen to what I was saying. She told me the HTS ComfortX3 was a reboarder. I wanted to say, "Yes, I know it is the one Gr1 seat HTS sells that only forward faces, but it looks completely different than the others how can you get them confused??" She told me that my youngest can't ride without the 'cushion' yet (on the cybex pallas) so she wouldn't recommend a booster. Um yea, I know that, which is why I was looking at Gr1 seats, not Gr 2-3 seats for him.
Another associate was trying to sell a different cybex seat (maybe a Juno) to another couple who remarked their child can't sit fully upright on his own yet.. but to his side, I have to say at the very end after they said, they don't want to rf because "2months forward facing on vacation and he just doesn't like to" the associate did recommend sitting rfing for as long as possible. I wanted to step in and say "what about the new norm that says at least 15 months rear facing."
Anyway, I wish my youngest didn't get so sick rfing so all this would be easy. I never believed people until he started puking all the time only in the car. :-/