I travel with our 2&4yo DC. Sometimes they are great, sometimes not. Same as at home really. But the way I see it is it's literally 2.5 hours. Who cares if they don't behave perfectly. As long as they sit when the seatbelt signs are on (large box of favourite snacks) and don't kick or encroach other peoples space, then it doesn't matter if they can't sit still for the whole flight. Kids make noise, they just do. Try your best to keep her occupied, that's all you can do.
I use the iPad, then wrap small presents, enough for 1 every 20 mins or so- stickers, window clings, play dough, puzzles, match/snap cards, books, colouring books, sparkly pens, phonics flash cards, plastic tat, things that come in eggs, fidget toys etc.
Limit sugar if it makes her hyper but constant snacks is the way. They have their own backpack filled with presents and snacks plus a new soft toy for comfort.
We always book a table in a restaurant before we fly so they are well fed with a hot meal. Mine act up more if they are hungry, plus it passes time. Then we go and have a long walk and choose a magazine (with more plastic tat attached).
We've done several 10+ hour flights now and I hope for the best, but know it's soon over if not.
My 4yo has the energy of 10 children and selective hearing,but is actually a pretty good traveller so you might be surprised.
If she's under 20 kg you could maybe use a cares harness. Just check with the airline.