Op there is a saying that I came across: I would not change my child for the world, but I would change the world for my child!
My dd 11 has ASD, learning disabilities and sensory processing difficulties. What helped us in the early days was, not taking dd out to busy places that could case a meltdown, avoiding them.
Noise cancelling headphones or ear defenders, maybe having a tablet or i pad and putting their favourite programmes on it to distract.
If they are in distress and having a meltdown, removing them from the situation asap, that means wolfing down your meal, and going so it lessens the distress on them, and also does not disrupt other people.
Maybe have Ds wear an Autism awareness t shirt, somebody I knew had business type cards made, and handed them out to people when he had a meltdown so that they knew, and as part of autism education.
We found that things got easier when dd was able to speak properly and verbalise her distress, at about 5/6 and the meltdowns significantly reduced. As she got older her understanding and tolerance to different situations increased which made things a hell of a lot easier.