Should still be enough the mini grafts don't have a very long lifespan so 75cm should be big enough don't underplant in the first year though Idid a mini avenue of potted mini apples and pears in a garden I designed once 5 years on they are still in the same pots and fruiting quite well
Yes, I have four on my roof terrace, 2 Pear, 1 each of Plum and Cherry. The fruit tree expert at the nursery advised me to get the 60cm cube as a lifetime home for them