Haha, I already answered your other thread but will post here with seats I've actually used with my now 12yo (who doesn't fit in any seat any more). The other recommendations are based on me having come across and compared them at work.
Britax Adventure - great, loads of room, on the cheap side (quality wise) but a good brand I trust.
CBX solution booster - terrible, too short, avoid. (Cybex = good, CBX = bad, no idea why, they are the same company).
Diono Hip (they don't make this any more) - really comfy, tall. Would trust Diono boosters in general.
Generic backless booster - fine - brand doesn't really matter.
There is a benefit to keeping using a booster with back BTW, it offers them some side impact protection if you're in a crash and it helps support them in the correct position if they fall asleep, which granted is rare for a 7yo, but did happen with DS1 when we were on a very long drive across Europe.
Some boosters you'll reach a limit where you can't raise the headrest any more. Commonly with Maxi Cosi, Graco, and some of the 123 seats with a harness you take out when they get to 3/4 years old. Some others go up higher. It's a bit trial and error - if you are able to try them in a shop I'd do that.