I can't help much with what seat to get, but I'd definitely go for a convertible booster. We bought one that's from 6m-8y and I honestly think it's the best way to go (not from 6mo though, we bought it when we bought our second car which was when dd was nearly 2)
It has the seatbelt harness like a car seat which you can take off to convert to a plain booster, but the seatbelt holes seem to go higher than they would in a car seat, which indicates you can keep them in the 5 point harness for longer, huge plus IMO!
We thought it'd be such a waste getting her a car seat which she'd have to be out of within 2 years, but obviously she couldn't go in a booster yet, so that became our best option.
I can't remember the name of our one, but it is rather bulky anyway, but they did have plenty of different ones when I was looking