Car seat transitions- DD1 to HBB or LBB?

[zag95]

DD1, age 7 is 52 inches and rides in BR Frontier (harnessed) ; DD2, age 2.75 is 40inches and 40 lbs and rides in a BR MA. These are the current seats in our ODY.

In DHs Honda Accord (NO LATCH), DD1 rides in a TurboBooster, and DD2 rides in a BR RD 50 or 55 (inherited from a friend- no accident)

In Grandparents' cars Volvo xc70 and XC90- DD1 rides in a BR Frontier harness and DD2 rides in a BR RD 50 or 55 (inherited from friend- no accident).

DD2 is at the top strap in the BR RD. I am researching what our next options are.

How would you move the seats and what would you get?

Thought we would do DD2 into the Frontiers..... and then have DD 1 in the HBB (Graco is one we have) plus another- is there a recommendation on best option? Both kids are tall- so want something that can last for a while (not outgrown due to too small a fit/seat width, etc.)

Questions: do we need HBBs, or can we just do a LBB for occasional riding? Kids primarily ride in the ODY. Is there a good LBB that is cushioned that we can get for our use and/or recommended in case we have a friend ride?

Thanks for your input!

[Joolsplus2]

Yeah, I'd get a nice highback for the main ride, and a backless is totally fine for a spare. The Chicco Kid Fit at Target is super cushy and fits kids well, even becomes a cushy backless later. Or the Britax Parkway, but the new G1.1 isn't quite out yet (it has better lapbelt positioning than the previous models). Sunshine Kids Monterey is coming back, that's been a fan fave for some time (went away due to factory issues, not because there was anything wrong with it). Recaro Performance Booster is really nice and big, but should work in a captain's chair.
A good spare for a taller kid would be the Graco Affix or Evenflo Amp (both backless), they weigh a little more than the super light Harmony (at Walmart) or Top Side (at Target), but have deeper/wider seats (my 9 yo fits but complains about the small size of those little ones).