For regular iron ore, if you can get your hands on a small piece of iron ore (drops on regular earth elementals, ) you can put that in your pack horse and then stack 400 iron ore vs 133. The small pieces weigh 4 stone versus 12 on the regular sized piles. I know this isn't quite the 600+ you are requesting, but it is closer. Unfortunately, I have not come across any small pieces of any colored ore, so we are currently stuck with 133 on those.
I was under the impression that a beetle has the same 1600 stone weight limit. Is this different on UOF? I have long since retired from mining in large quantities, and when I was mining, beetles were selling for well over 100k, so I never acquired one. Since you can't mine while mounted, I am not certain if there is much benefit to the beetle unless it can carry more weight.
True, as the mounting and dismounting of non-ethy mounts is instantaneous. And you don't have to worry about it doing stupid shit, getting hung up on some easily avoidable obstacle like packies invariably do.