We recently had a similar dilemma. DS2 refused to sit in his seat S&S Premium AHR & wanted to sit in DS1 S&S maxi. To solve this we transferred DS2 Maxi from DH's car for DS1 but it seemed smaller than the older one & true to thought when DS1 sat in it the seat belt was left with about 6 cm of room left to play with. We didn't want to mess with the original as it is properly fitted and DS1 is quite tall so decided on buying him a full booster seat anchored in like the maxi but using the car seat belt. DS1 is about 17 & 18kg & DS2 15kg. Working well so far.
If you decide on a maxi rider I have a spare. Hardly been used as it was in DH's car & DS2 rarely went in it. I would say roughly 10 times for both boys & no accidents whilst it's been fitted in. I'm in the eastern burbs.
Not sure if anything I have said has helped. Hopefully it has.
Thanks we are out west, but will let DH know. So hard to know what is right and wrong to do!!