I would get a spinning seat up to 18kg (or 20kg, doesn't really matter).
I don't think she's likely to get as good a fit in the 25kg ERF seats, you don't need the capacity, they are nicer for leg room for longer term RF, so that's a positive if you wanted it, but other than this I probably wouldn't bother with them with a smaller toddler.
Most people like the spin function and most of the spin seats are comfy to use, have the option to recline when she wants to nap but sit much more upright than the ones on the toddler bases.
I would skip any of the all-stages ones. They are a false economy, they aren't as safe and they are more bulky and less tailored to the stage she is in now.
The Sirona Z2 (Surely it will be the original Sirona Z that you need? I didn't think the Cloud Z2 was out 2.5 years ago) is fine, but it's a £300 seat with no special features, and there are loads of other spin seats under £300 so I wouldn't bother with this. Keep your base free in case you have another baby, or to sell on with the baby seat if you're not. The only time the Sirona Z might make sense is if you frequently switch cars and already have a base in each car.
I'd go to a local baby store, an independent if possible, and try out the different models. See how she fits in them, see how they are to use and adjust, when there's one you like, trial it in your car. There's a couple of Cybex ones which don't need the base - the Gi Sirona as you say, or the Sirona S2. Maxi Cosi have the Mica or Mica Pro Eco. Britax have the Dualfix. There is also a Dualfix Plus which is up to 20kg as well. Joie 360 Spin is also popular and Joie i-Spin 360. All of these are under (or around) £300 which is the same cost as the Sirona Z.
You will need to keep her in a 5 point harness until she is 15kg and 100cm, as these are the minimum requirements for a high backed booster seat. There is some evidence that children are better off in a high backed booster until they are closer to 18kg and 105cm, but this is probably related to age and might not apply in your DD's case. However given that she is likely to go into a high backed booster on the lower end of the size ranges simply because she will be that much older by the time that transition comes, you are going to want a dedicated high backed booster and you're going to want to try different models at that time to see which one fits her the most securely. So that's another reason to seek out your local independent nursery specialist :)
A good way to find an independent store is to look up a brand (like Cybex) and go to their stockists page, put in your postcode, discount all the chains like Mamas & Papas, John Lewis, Halford's, and see if there are any names that you don't recognise - try there.
In the absence of independents, John Lewis seem to be the best of the chain stores.
Good luck!