Vol 7. hasn't been released, true. However, the volumes printed so far collect through the January 1987 cover dates. The next books to follow that are THE WORLD OF KRYPTON, which was reprinted not long ago. So, that serves as a sort-of Vol 6.5, if you're interested in more books from that era. Following that is MILLENNIUM. The mini-series was recently reprinted, but none of the Superman tie-ins.
Sadly Millennium was my first crossover after my intro to DC, which was COIE. I purchased all the Milenium crossover books, all 42 or so. I think I got suckered on legends as well.
Its probably cheaper to buy the back issues I'm missing at this point...