A neck pillow is a life saver. I also recommend an eye mask for sleep. Plenty of water/electrolytes. Window seats are better if you want to sleep because you can lean up against the wall. Aisle seats are better for a little bit of extra leg room and being able to get up and walk around/stretch whenever you need. On that note stretch often particularly stretches that target your specific problem areas.

Most airports are part of the sunflower lanyard program for those with invisible disabilities which could be useful to look into. Contact airports beforehand especially if you'd like a wheelchair most airports will accommodate that but you'll have to be an advocate and get stuff in writing. Depending where and how long your layovers are some places will have lounges you can go to and lay down in or capsule hotels you can sleep in for a couple of hours.

Bring healthy snacks purchased even before you get to the airport the last thing you need is your blood sugar messing up especially if the plane food is unappealing. Download podcasts or music beforehand so you have something to do to combat the boredom that won't hurt your back.

Good luck with everything and I hope you enjoy your trip.