I’ve never tried managing peaches that way but E and J plums have done OK. You just have to keep pruning them all summer on about a biweekly schedule.
I expect that the shaded growth got adequate light to produce next year since the shade was not during the majority of the growing season, right? What I’d do is prune half the trees and leave the other half and try to really learn something. If I’d listened to Cornell, I would have thought the only safe time to prune is in late winter and early spring.
A sealant probably won’t help you. For most situations they’ve been debunked for decades, starting with the work of Alex Shigo. Most do more harm than good. Lac Balsalm may be an exception- at least it appears to do no harm.
I had a devil of a time keeping my own nursery peaches adequately pruned this season. Growth was rapid and relentless and the trees are much too closely spaced. The deer must think I planted the trees as a crop for them. They won’t eat the lower branches I’ve sprayed with repellent much but every branch I cut from higher in the trees has every leaf stripped off in a day or 2.
I have continued pruning peaches here- just the most vigorous shoots, even on my 2-year nursery trees. I expect there is plenty of time and energy for your own trees to protect themselves from injury you inflict on them this coming weekend if you just do this. I’m just guaranteeing nothing. A few big thinning cuts shouldn’t stimulate a rush of growth- even if that was an existential issue.
One year we had a long summer drought that broke in the last week of Aug which caused a late surge of vegetative growth- especially on J. plums. A very cold winter followed and a lot of that new growth was killed but the trees themselves were fine. It was only the new growth that failed to adequately harden off. I’ve never seen this described in the literature, which always suggests that a late surge of growth is dangerous to the ENTIRE tree.