My kid was large at 2.5yo so we just used the regular seatbelt. Rented a car at the destination with a carseat if we needed.

If you don't mind checking the car seat or renting one at your destination, I found this product that looks really cool! I'm planning to get them for an overseas trip this summer, and one of my kids is the same age as yours. In my opinion, it's much easier and more comfortable to sleep when you're laying flat: https://jet-kids.com/shop/jetkids-bedbox/

if you anticipate flying with her in the future I would consider a different lightweight car seat than the cosco scenera next (unless you already own it) She's almost outgrown by height.

I'd consider the evenflo sonus which is only 11 pounds (just slightly heaver than the scenera but with more generous height and weight limits.

Haven't used the care harness so no opinion about what is better.

Definitely a cosco scenera if she still fits. The CARES harness she will just flop and have absolutely no support to sleep, it is not great. it's one of the biggest downsides of it in my opinion but is fine for just rental cars between places when traveling. But I wouldn't make it a plan for sleeping overnight in a plane.

We are flying across the country with a 2.5 year old who is in the 95th percentile for height. Our flight is late so we expect her to want to sleep. Would we be better off with a cosco travel carseat o the plane, or a cares harness?

Appreciate any thoughts! Cross posting in the travel forum.