Depends if you would be happy with a booster seat or not. At almost 3 it's borderline. Some would say you want to wait until nearer 4 to put them into a booster. I might be tempted to do it with something like the Britax Kidfix with the crotch strap which prevents them submarining under the belt. High back of course, and I'd look for one with excellent safety ratings and try it out in the shop with the child to see if they would fit nicely into it and be sensible (not play around with the seatbelt etc). The manufacturers seem to recommend 3 as the minimum age for a booster seat. However, some parents aren't comfortable with putting their 3yo into a booster - and at 16kg/95cm, you'll probably get about another year out of a harnessed seat, if that's something you'd rather stick with.
Otherwise, what you're looking at is either two seats - one for use up to 18kg, seatbelt fitted with harness, and then a separate booster seat for after that - or what they call a 123 seat, which is harnessed until 18kg and then you take the harness out or tuck it away, depending on the seat, and it becomes a high back booster. The main problem with these is that they do not tend to crash test well (Which? say this is to do with them trying to tick too many boxes) and they tend to be heavy, whereas a booster seat in particular can be very lightweight. I reckon you'll get about a year out of an 18kg harnessed seat, possibly a bit less.
Anyway - separate seats I'd look at:
The harnessed seat:
Joie Tilt/Steadi - neither have been tested by Which? but both have the possibility to rear face, which automatically means better safety than most of the Group 1 seats (if you use the rear facing feature, which you could - it's still safer, even at 3). The Tilt is slightly smaller, and weighs about 6kg whereas the Steadi weighs nearly 8kg. If you can check your child in the Tilt before buying, do, as it's quite short in the shell.
Britax Eclipse - if you don't think you'll use the rear facing feature, this is a bit cheaper, and it's more versatile as it can also be installed with just a lap belt. It was reviewed by the same testing body as Which? in 2010 and received average results. 7kg.
Then a booster later - Joie Trillo Plus is a clear winner as it's cheap, lightweight (4.5kg) and safety tests very well - better than any 123 seat.
In the region of 123 seats, Britax Evolva is probably your best bet - not the lightest at 8kg, but gets quite good safety results compared to other 123 seats, especially the more enhanced versions (SICT, SL, etc). Affordable too.
Specifically not recommended:
Joie Elevate - gets a poor fit in many cars
Anything own brand, Nania, Cuggl, Team Tex, generic, Harmony, Migo, Cosatto, Kids Embrace - these are all known poor performing car seats and especially so for the 123 models. There are more "rebrands" of these than I can name but essentially if it seems too cheap to be true, it probably is. Britax and Joie both have acceptable budget models.
An impact shield seat would seem to tick all your boxes - Cybex do some, and Joie - and that is exactly what I used in your position 8 years ago. They crash test well and are lightweight and non-isofix - BUT - the data I've seen since then makes me wary of recommending them for a child at the top of the weight/age range for the impact shield part. They just don't seem to perform well for these children, to the point that I actually think (note this is opinion and I can't back it up) they would be safer in a well fitted high backed booster seat. I did move DS1 to a booster seat at 3, not because I thought the impact shield was unsafe but mainly for convenience.
The wildcard I would suggest you look at would be the Joie Every Stage, or Joie Verso. It's a wildcard because Which? specifically states that it's a don't buy - but this is because it receives very poor safety results when tested as a forward facing seat with harness. You could use it in rear facing mode until 18kg, and then change to high back booster mode, and it performs OK (but still not great) in both of these modes. However it's heavier and would probably be more cumbersome than the Evolva.