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Inspect the tree carefully.
Do you see any other evidence of insects or bugs?
Chewed leaves?

If so you could use a insecticidal soap or Neem Oil.

I have heard that squirrels will nibble on peaches. Do you have those critters in your garden?
I have garden friends who actually place Squirrel feeders near the trees to encourage them to stay off of the fruit trees,

This would be a more difficult task to control.
Birds also will eat your peaches.

Here are a few links.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

https://www.gardeningknowhow.com/plant-problems/pests/animals/get-rid-squirrels.htm

https://www.gardeningknowhow.com/edible/fruits/fegen/squirrel-proof-fruit-trees.htm

https://www.gardeningknowhow.com/edible/fruits/fegen/protect-fruit-tree-birds.htm

The leaf yellowing is due to stress. Since you mention the ants and flying insects around the tree, my guess would be that is has a pest of some kind, likely aphids or a similar sap-sucking insect. They generally tend to secrete honeydew, a sticky substance which may be present on the leaves. This honeydew is what attracts the ants and flies. I would recommend treating the tree iwth neem oil, which should take care of the pest problem and in turn take care of the ants and flies. It is also safe to use and an effective fungicide as well should other issues pop up due to the stress from the pests. It is often sold as fruit tree spray at stores. Check the label. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm

It sounds like you are getting powdery mildew on the fruit. This article should help you: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm

Proper watering is usually the cause of newly planted tree issues.
Here is an article to help you with the care of new trees.

https://www.gardeningknowhow.com/ornamental/trees/tgen/watering-newly-planted-tree.htm

Typically, in order to get larger peaches, the trees need to be thinned. Deciding when to harvest peaches is a matter of personal preference. While most people prefer tree-ripened fruit, but fruit at this stage has an extremely short shelf life. Therefore, it may be better to pick the fruit two to four days before it is fully ripe. Here are some things to look for when harvesting peaches: When the fruit is easily pulled off the twigs then it is probably ripe. You should feel a certain amount of movement when you press in to the flesh of the peach with your fingers. When peaches change from green to an orange tint, they are nearly ripe. Ripe peaches will give off a pleasant, peachy smell.

These articles should help you with growing and harvesting peaches: https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-care.htm
https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-thinning.htm

It does sound like peach tree borer.
Here is a link with more information and how to treat your tree.

https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-borer.htm