Thanks. He said its 4 - 4.5mm, which isn't the thickest, but he said if I went thicker it may feel like a hoop. As far as hardware goes its a preference thing really, and all about what you like/ want. Importantly I wanted brass or copper rivets and as far as I know these are hammered like they were in the old days- apparently this is the strongest possible method of riveting. I also like the 'hammered look, but cant take credit for it, the leather smith did it. Chicago rivets are a great idea, but IMO they can come lose, or at least they use to 20 years ago, not sure about now! But I do believe they are not as strong as a hammered/ fixed rivet.
I looked into Hanks Belts USA, but even their high end gun belt with heavy duty stainless steel buckle and 5.5mm thick bridle leather, only had nickel plated steel rivets (Chicago rivets). But I wanted rivets that would last and had character. I considered copper, but chose brass. According to YouTube brass lasts longer, but copper is meant to be the strongest on leather, but I wasn't convinced so I chose brass. From my experience, once nickel plated steel is scratched it starts rusting. These are the reasons I went for the rivets and materials I did. The other belts would probably last too, but I'm just a perfectionist I guess and I could be wrong too!