I think 1/2" thickness is strong enough. I used 3/4" when I made my Standard Gauge layout and it was overkill. Trying to get 3/4" 4'x8' sheets down to my basement was a pain. They don't flex much and are rather heavy.
I went through this same issue several years ago. I was making changes on my barn and wanted to replace the original 4'x10' sheets of T1-11. I know they are produced but I just couldn't find a place that had them. I was willing to travel too.