In Florida she actually legally needs to be in a 5 point harness until age 4 anyway.
How often will it be used going forward? You're looking at 123 style seats, but these are so hugely variable in their crash performance that you want to be careful with them. I also think they are potentially more prone to damage in the hold of a plane - I'd recommend keeping the original cardboard box, fold it flat in the car, bring a roll of parcel tape and then either children's craft scissors (chuck into another hold suitcase) or cut it with your teeth and split with a key at other end. Or you can wrap very securely in a roll of bubble wrap. The car seat bags you can buy don't really offer any protective padding.
It sounds like you'll use it more as a booster seat than a harnessed seat, long term?
Luckily she's well under the weight guidelines for a standard one (up to 18kg) and you don't need the extended harness (25kg).
I'd probably look at Recaro Tian - it is seatbelt fitted for the harness stage, isofix for the booster stage, lighter than some other 123 type seats and very good safety in both harness and booster mode.
If you don't want to spend this much, then you could look at Joie Elevate - probably the newer R129 version is best. This is harnessed up to 105cm and then can be used as a high backed booster. No ADAC crash test rating on it, but Joie have well rated boosters in general. Their forward facing harness seats haven't historically done as well in the ADAC test, but the R129 approval is a good thing. This is more basic, and is essentially a high back booster seat with a harness attached (whereas the Recaro is more designed as a harnessed seat which converts, making it bulkier as a booster seat, but more protective as a harnessed seat). It's belt fitted and lighter than the Recaro.
For isofix installation for the actual seat itself, I think Maxi Cosi Titan Pro or Britax Evolvafix are best, but both are heavier and bulkier than the other options mentioned. Maxi Cosi do also have another option called BebeConfort Everfix, which is their budget line, for about £150 - if you're going to use a cheaper brand, then this is a good one to use though it's not realistically that cheap.
There are very cheap isofix 123 type seats e.g. Kinderkraft Safety Fix 2 i-size, but I don't know how well these are made and how well they would perform in an accident. They have isofix and top tether, which is a plus, but Kinderkraft have previously got quite bad results on the ADAC test, so that's a negative. I'd be wary (though it will certainly be better than nothing!)