Maestro

Cosco Scenera Next and once they grow out of it, Evenflo Maestro.

All carseats have to meet minimum side impact standards in the US. You can read independently tested carseat ratings on Consumer Reports.  I did a fair amount of research. The story that all the consumer ratings don't tell is how well they perform real world for a city dweller using a cab or rideshare. Installing the carseat quickly and correctly is different in a city setting. The best rated carseats are designed to be installed slowly in a vehicle that you own, the opposite of what would happen in a cab. The Graco definitely has a great safety rating but it will not matter in a cab if it's not installed perfectly. That's why the Scenera Next is a favorite amongst city dwellers, it's easy to install correctly and has a good (but not the best) safety rating.  The Maestro is for later when your child is 3 (or in my child's case 2.5) and they are too big for the Scenera Next. Unfortunately there is no perfect carseat for the city when your kid is too big for the Scenera Next but not old enough for a booster seat. The Maestro is the lightest I found with an average safety rating.

Everyone has covered the key parts of your questions so I will just add: Definitely get a travel car seat. They are much lighter and a little narrower but still safe and compliant. We had one from Evenflo called a Sport or something but if you search on Amazon for a travel car seat options will come up. Extremely easy to install.

The buckle position will be the same on the airplane, but Evenflo allows padding it with something small like a jacket. Maestro is a great choice for a tall child who isn’t going to max it out by weight first, the top harness slot is much higher than Finale so they won’t outgrow it by shoulder height. I am a Child Passenger Safety Technician. 🙂

With those stats I’d recommend Evenflo Maestro as a replacement for Cosco Scenera Next - if lightweight is the priority. If a little more weight is acceptable, Graco Tranzitions will also convert to a booster and its headrest is adjustable to ensure a good fit. Just be sure to buckle up the harness when carrying it through the airport. It can come apart to backless booster mode and you don’t want that happening by surprise in the airport.

Look at Evenflo Maestro for travel

Evenflo Maestro is lightweight and fits those stats. I am a Child Passenger Safety Technician (CPST) and recommend it often for travel.

Put kiddo in a lightweight forward facing car seat instead, fits better and feels more like the car so kids behave better. Then use at destination. Cosco finale or Evenflo maestro is super light.

Check out the Evenflo Maestro! It’s a forward facing harnessed booster seat and it’s really lightweight. I just traveled with it for my kid who is exactly the same size as yours.

I think you answered your own question, but if by booster, you mean a car seat without a harness that uses the vehicle seatbelt to hold her in, no it would not be okay since she doesn’t meet the minimums (age 5 AND 40lbs AND the ability to sit correctly at all times) of any booster seat. She needs to remain harnessed in a convertible or combination car seat. Since she’s under 40lbs, she’d also be safest riding in a harnessed car seat on the plane as well. If you’re looking for something lightweight/packable for the car only, the ride safer travel vest is basically your only option. My 5.5 year old is basically the same size (37lbs, 43in) and we just bring her normal car seat(s), graco slimfit and century drive on, when we travel by plane. If your normal car seat is bulky and heavy you could purchase a more travel friendly one like the Evenflo Maestro or the Cosco Finale, but make sure you measure her torso height to make sure the harness height is compatible.

I have a rotating one in my girl and a regular one in my husbands. We both prefer the rotating one!

My skinny kids get bruised from some seats. Ive also had a cheap seat that was so difficult to tighten i threw it out. If i couldnt guarntee i could fasten the kids safely, it wasnt a safe seat. I will never touch the Evoflo brand again.