I think there are too many different kinds of disks as there is.
Should just simplify it.
First, the more bulk you carry, the slower it goes until it won't take any more at 0 speed.
Second, all of them have 10 stacks. I never saw a reason to force them down to one or two stacks, just to increase bulk capacity. Makes no sense to me. If the thing can carry 12,000 bulk of one kind of material, there's no sensible reason it can't carry 1,200 bulk each of 10 different materials outside of the need to make an artificial game mechanic.
The only other options would be "can fly with", "can portal with", and "can recall with", and tiering would just mean a higher bulk/speed ratio so you can carry more or faster.