It is a minefield indeed!
Have you checked whether there are any infant inserts which can be removed from the seat you have, and have you extended the harness and/or headrest to keep up with her growth? Sometimes people don't realise you can do this and it helps keep the baby comfortable in the seat for longer.
If you do want a new seat, I would ask you if you want a seat which you can use until she is ready to go into a booster, or whether you're happy/prepared to buy another seat (in this expensive category) before you look at booster seats (which are cheaper). At 6 months it sounds like you know this already, but she needs to still be rear facing whichever seat you choose.
If you plan to have a second child, then you will need to buy two toddler type seats eventually anyway, so it can make economic sense to get something like the Joie 360 for now with a view to upgrading her to a larger seat (the next size is up to 25kg) later when DC2 needs to move out of the infant seat. You can then postpone the decision for a while about whether you get a forward facing or rear facing 25kg seat.
However if you're unsure/definitely don't want another baby and/or want to make sure you get a seat now which will last her until she's able to use a booster, then you'll need one up to 25kg now, and as Tea says, these aren't really great for children who are 6 months old. Independent sitting, 9kg minimum weight and/or 80cm minimum height are all around the milestones you are looking for as a rule of thumb for these seats. You won't have the option of isofix installation and the recline has to be set at install, so you usually choose a more reclined position for a younger child and change it to more upright as they get bigger.
If you did want to try out a 25kg seat now, the two best options to look at for younger babies are the Axkid Minikid (new version with infant insert) and the Diono Radian 5.
If you want a 25kg seat long term but don't want to get it yet and don't want to keep using the baby seat, another option would be to get the Joie Tilt as a stopgap seat, this is about £70, it can be rear facing up to 18kg but has a better recline and is smaller in size so better for under 1s than the 25kg seats.
I have a tall 18 month old in a Joie 360, he is currently on the third harness/headrest slot out of six, he's about 86cm tall and 12.5kg. I'm happy with the seat for now and I like the fact it has recline options on the go, which the larger seats don't. It's meant to be one of the tallest of the seats to 18kg. The Axkid Modukid is another seat which is fairly accommodating to bigger children while having a limit of 18kg (meaning it can recline and use isofix and be suitable for children under 9 months or so).