V70 built-in child seats vs. retrofitters

On the other hand, I drive an 11/04 V70 D5 with the two built-in child seats (15-36kg) and side airbags. Our taller (2 3/4 and 17kg, rather a bit “robuster” has now grown out of his Roman seat recommended up to 18kg and we are still not quite conclusive whether we need a successor at all. On short distances he sits on the built-in seat cushions and feels comfortable on it. On longer (motorway) distances we still squeak him into his old seat. About the search I found already threads dealing with the fit/fit-up (headrest) of these 15-36kg seats. Volvo itself has not any of these seats in the program. Our market analysis brought 4 seats from the to light: Concord, Römer, Sybex and MaxiCosi. When proposing we could not “seat out” any favorites. Question: with which seats did you have good/bad experiences. Why Volvo does not offer a seat in this class? Are these seats really meaningful or protects the built-in-in-in-a-seatted thanking bag or better?