I don’t see this as being very different from my current building, where we put storage cages in unused common areas and then rented them out to owners. The collective benefit to the owners corp was the income from an otherwise unused common area. It was all mostly legal until our popinjay chairman decided arbitrarily that when they voted on raising the rental of the storage spaces, people who were renting them were excluded from the vote while those who were on the waiting list for storage were allowed to vote – all of this in the interest of “fairness”.
I think ‘differential levies” is a minefield that you really don’t need to enter. It may not be legal (or at least legally tested) but I wouldn’t fix it until someone complains that it’s broke.