Here’s a tip you probably won’t find in the literature that I learned from hard experience. Read the article I posted on roots and realize how competitive established large trees can be to young transplants- even when they occupy the north side only- and are twice as far away from the transplant as they (the established trees) are high.
When digging holes for a transplant, any roots severed from nearby trees will send out vigorous feeding (and drinking) roots from the spot they are cut- just like a stub-cut branch sends out a riot of growth specifically at the point of the cut. These feeder roots will be encouraged by the soft, oxygen and water rich soil and quickly occupy the soil your new tree is trying to get its “footing” or rooting in.
Peaches are especially poor at competing with such roots but I’ve had pears runt out that have been planted near large forest trees as well.
Extra water and fertilizer will help but sometimes you need to dig an especially wide hole and sever competing roots with a heavy spade (18" deep) at that holes edge in mid spring for a couple of years to help the transplant become established if it is facing excessive competition.