I’m an autistic adult and have travelled a lot to the US (have never had a problem taking my safe foods).
you mentioned flight tips … obviously everyone is different, but some of the things that get me through are:
Having a seat booked at the front or back of the cabin - less overwhelming than being in the middle.
I like to board last to minimise the noise, movement, general chaos of everyone getting settled around me.
after several disastrous trips and at the suggestion of BA I now declare my autism, which has been helpful. I have a sunflower lanyard. This also often allows for a quicker time at security, which massively lowered my meltdowns.
During more difficult times, I have on occasion had to use a wheelchair to get through the airport. While I feel embarrassed, it actually really helps.
creating a written schedule for the flight. Ie, what time I will snack, what audio I might listen to and when, when I will take a walk around the cabin… this gives long haul some structure and makes me feel more in control.
ear defenders and noise cancelling headphones. I use defenders is people are talking loudly and the cancelling headphones if it’s just white engine noise.
some airports have special assistance waiting rooms or even designated ‘quiet rooms’. These are really helpful as the less drained by sensory overload I get in the airport the more manageable a flight is. Failing this I usually look for the airport chapel or prayer room as the area here is usually calmer.
trying to minimise exhaustion prior to the journey also helps as I have more room to process sensory information.
similarly the more I know about the trip and schedule when we actually arrive the better as I’m not then dealing with too much uncertainty about destination and can deal better with travel.