2 to 5 foot depth in proportion to horizontal dimensions for lined ponds. Earth bottom ponds may be much deeper.The real key to moderating the water temps in this area is shade, shade, shade. which of course usually means trees. Just to give you an idea of the difference, 4 hours of sunlight in my area is equivalent to a full days sun in your area. This was determined several years ago in a Horticultural study to determine just exactly what the term 'part-sun' meant. If you are a gardener or landscaper you know what I am referring to. The temperature of the water column in a pond, in this area, can easily exceed the ambient air temp. This becomes more pronounced if it is a bare liner pond; the liner is black and black absorbs heat.
Water changes are good. I am inferring that you have a well. If so, you might want to have it tested periodically. Well water, besides having very low, if any, oxygen, may contain other harmful dissolved elements and/or hydrocarbons that could be toxic to your fish. I once had a customer who went through a ton of money replacing Koi that kept dying. He was using well water. After much prodding by me, he finally had it tested and found that it was very high in Copper which is extremely toxic to fish.