I didn't read every post, but I would very strongly recommend using a rubber liner, and disregard the spring other than perhaps a nearby shallow well with a small pump in it for a source of water to keep it topped off. A few reasons why:

1. Initially the ash tree will rob you of water. More than you would think. Roots will regrow and they will be more than happy to grow back into the pond and cause problems.
2. The Ash will die. Emerald Ash Bore. The roots will rot or the tree will fall with roots all on one side removed. You will have more places to water to go or you will get a larger unwanted pond with a large root ball in it.
3. Your island is robbing you of a great deal of water volume. Remove it when/if you put in a liner and instead build a floating island if you still want one. The island will provide a refuge for future fish from herons, and will provide some shade to keep the water cooler and fish happier.
4. When/if you put in a liner, it will look much better at the front of the house, and the rubber acts as a weed barrier making it easier to maintain. Of course this means covering the exposed line with stone.
5. You can clean it out FAR easier than a clay bottom. Drain it with a trash pump, scoop out the crud with a bucket and rake. Brand spanking new again. Especially important with tree leaves.
6. You can run the water right up to the same level as the grass if you wish with the rubber liner. Border with rock cobbles, build shelves in the basin that holds plants, etc.

Expect about $1K or so in cost though, and you will need a felt liner between the soil and rubber to prevent punctures.

I have a 1/2 acre field pond, and a garden pond a little smaller than what you show here with a rubber liner. I only occasionally (2-3 times a summer at most) need to top it off with a hose when it gets one or two inches down as our evaporation rates in NY are just a hair shy of our precip rates. Meanwhile, the farm pond will be down 3ft from seepage and evaporation combined.