I have given a lot of thought to this and think that it makes a lot more sense to have the the moons actual consist of all goo and mineral types with some moons having higher density of one substance over the other. Then rather then always spawning the same belt it actually produces random belts based on the probability for that moon. One time it could be all R4 and the next be intermixed with some R64. This reenables solo miners and smaller groups without hurting the major alliances. It also gives more reason to cycle the current R4 moons and would encourage more structures and more opportunity for conflict.
As long as the probability of spawning is the same as the ratio of R(n) moon goo then it shouldnt drastically shift the market and only allow it to spawn in different locations.
One of my favorite parts is this also fixs the mega multibox accounts that wipe R64 moons before anyone else can get to them. This is such a problem that alliances have resorted to hiding Fraks from players to give other players a chance rather than having one or two players control the r64 market.
For Metenox reduce the gas cost and then allow players to tell it to target types and then it produces based on that moons probability for that type. I.e. If i want vanadium/cobalt/dysprosium i can target it but would be much lower yield if i was on what is currently considered a r4 moon.