Sorry to hear about losing your big fish too.

Three foot deep water is hard to keep conducive for thriving fish populations during very hot weather.

Are there any other possible sources of water available on your property? You can pump water a LONG way with a little 2" gasoline trash pump from a creek or marshy area.

If no other potential sources, can you build a supplemental source? Does your water shed provide more than enough water to your main pond during normal/rainy seasons? If so, maybe consider building a water capture pond just downslope of your existing pond. I would make it as steep and deep as your sub-soils profile allows. (That would minimize evaporative losses.)

During a drought, you could pump that pond dry to increase the water level in your main pond.

You could always use the second pond for lower value projects like raising FHMs that you wouldn't mind losing when you pumped it dry.