Here in Oregon, the Willamette Valley in particular, I’d recommend waiting until late spring before cutting it back to the 30 inches you described. Within the bark are ‘latent buds’ waiting to sprout if the tree is damaged. Your homework sounds right, as a ‘vase shape’ is the typical form for a peach tree.

As the tree would be dormant, you’d not be weakening it by removing the upper portion. In fact, so much root is lost during the ‘harvesting / transplanting’ process for sale it’s a good idea to remove some of the upper portion to balance it out with the root system.

My only question is when is a deciduous fruit tree dormant in Phoenix, Arizona? If someone else can tell you/us when that occurs, right before it begins its next season’s growth would be the time to prune.

A major problem here in the Pacific NW are the multitude of infections that attack peach trees, so leaving an open wound for any length of time is not a good practice. If you were to immediately paint or seal the cut, which is basically going to leave you with a thirty inch ‘pole,’ that should adequately protect the wound. As the weather warms and growth begins it should push out several new shoots very near the cut. These will become its limbs. Protect them!

Otherwise, keep us posted

It’s advised ‘up here’ to prune peaches after they’ve bloomed, to keep infections to a minimum as the flowing sap will immediately seal/heal the wound. In your case, you don’t want any stored energy going into the upper portion of the tree, as you’d be removing it.

Plant it now, that will give it’s roots the opportunity to ‘dig in.’ And ‘to you,’ this is likely “late Fall.” I’d lop off the top in late January, perhaps sealing the cut with something meant for that purpose – though it seems a shame to buy a quart of ‘tree heal or seal’ for just one cut. Interior latex pain dabbed over the cut would likely work fine.

Good questions, good luck, and you’re welcome