Two years you had great tasting fruit, and this year sweet mush? Ventura California? How has the weather been different this year than the last two? Seems like you've been in drought for years and it let up- barely- in the last year. When did the irrigation line begin to leak into the tree's
root zone?
I haven't experienced the texture problem, but it's possible it could be a soil thing, a lack of fertility, lack of the right mix of what the tree needs. I'd focus on how this year differed in climate and soil conditions.
The irrigation leak could have brought more water to the tree, but if the tree looks healthy too much water might not be the problem either.
The manure could have been nitrogen rich, which would get a lot of foliage, which would normally make the sugars and other compounds that are distinctively peach.
Factors to consider, and forgive me if you already know this, it's just my methodical way, and by reviewing the situation I might discover something I was not aware I knew. I'm just sort of thinking out loud. It keeps my attention on the question at hand.
The tree creates primordial buds in the fall before dormancy, up high in the sunlight. If you prune too heavily, you can cut off all of next year's fruit. The tree won't make new flower options in the spring. Late frosts can thin the fruit.
After flowering and fruit set, the tree puts out its new growth. If the leaves are deep green and the "normal " color for peach leaves it's a good sign. If the tree grew a lot of lush foliage and ran itself short of minerals, often times the color of the foliage is off, or the veins or margins are the wrong color. I don't think too much foliage would cause flavorless mushy fruit.
What's happening in my orchard this summer might be going to yield a deluge of flavorless soft apricots. They were ripening just fine, and at the regular rate. Suddenly it turned very hot and has stayed that way. In the heat the fruit will soften and drop, but it won't be bright with flavor. It's like they'll get beyond prime from the heat, before they get their sugar up, and their flavor. If I call sweet tangy flavorful "ripe", and soft and dropping from the tree "mature" then what is happening in my orchard is the ripening process and the maturation process are not synchronized.
I just re read the original post and it seems the heat might be doing what I think is happening in my orchard, It is just my theory, having watched my apricots fast forward, and knowing -thinking I know - what's in store. And I agree, it is heart breaking, especially considering that we have not had apricots since 2010. And I have not lost a crop in this way before. The "normal" way to lose a crop is the late frost, or a normal frost after an early warm spell.
So sorry for your peaches. I at least have peaches nectarines plums grapes and apples to follow. Unless we have another anomaly in our summer weather. Here's hoping!