Thank you for those who have added their input. I sincerely appreciate it. Here are a couple follow on points and responses to your questions.

I sent water samples to Texas A&M University for testing. Here are my results from a month ago, with results from April 2024 in parentheses, followed by the "normal" range.
Hardness ppm 55 (15) 50-150
Calcium ppm 15.6 (4.26) 15-45
Alkalinity ppm 17 (4) 50-150
pH 6.73 (6.22) 6.5-9
Salinity 0.04 (0.03) 0.5-3
Nitrite ppm 0.022 (0.01) <0.5
Total ammonia nitrogen 0.45 (0.47) <1.0
Unionized ammonia 0.001 (0) <0.25
Chlorides 20 (40) 50-100

Between the first test in () and the more recent test, I added 4,400 lbs of ag lime and 750 lbs of ag salt (non-iodized). I added less than the recommended amount of salt because that was all I could get from the local ag store at the time. The recommendations were 150-200 lbs/acre foot (about a ton for me). It looks like my efforts where worthwhile, but there is still some improvement to be made in the chemistry.

The geothermal is a closed loop system with coils that are buried under the frost line about 150' through my yard. They extend into the pond about 4' below the surface so they are safe from freezing. They naturally float so I weighted them down so they would sink. There is a coil of this plastic line that looks like a giant slinky, about 3-4' in diameter and about 10' long. This was weighted down with 250 pounds of ballast. It is in about 10' of water. I don't have a temperature of the surrounding water immediately around it, but I'll try go get it.

We're happy with the geo system so far and have had it running for a little more than a year. It is a new house with great windows, 2x6 walls and spray foam insulation throughout and the current indoor humidity is 38% without a humidifier. The geo furnace has a built in 220V electric heat back up that has never been used except during testing when it was installed. The house is as warm as we want it and costs about $300/yr to run when accounting for the solar panels on the roof that produce most of our energy needs.

I haven't tested the temp and DO at the bottom of the pond. I expect the DO is negligible. Where I took the measurements in the original post is about 10' deep but I could only sample up to 6' deep. That's from the top of the ice, not the bottom of the ice, so my 6' measurements were really 4' under the ice, since the ice is nearly 2' thick.

There is some flow through the pond even on the coldest days of the year. I haven't measured it but I would estimate that it's only about 10 GPM now. I have seen it running at 100+ GPM before and it will be again soon as the thaw starts.

I've tried a Vexilar flasher on the pond and didn't mark any fish. Given my DO readings, I was fishing too deep though. I've considered having someone with a side facing sonar come out and "survey" the pond to see if we can find anything. That may be my next move.

Regarding the tannins, my water looks a lot like other natural bodies of water near me including a very small pond connected to a stream about 1/4 mile from me that holds GSH and a naturally reproducing brook trout population. In fact, it is fed by the same watershed as my pond. I have noticed that this very small pond has a decent fish population during open water but I tried trapping minnows there last winter and got very very few. In open water it's easy to get a few dozen GSH overnight. I even caught a 6' BT that made it's way into a small Gee minnow trap.

Given that my pond is fairly new and therefore has very few aquatic plants, I expect I have very little O2 production (especially when most of the pond is under 16" of snow). I had some FA, however there are biblical numbers of crayfish in the pond so they seem to keep that in check. I intend to plant wild rice and pickerelweed this Spring for some emergent vegetation and fry refuge. Hopefully the phytoplankton and other microscopic plant life are producing some O2 through the ice.

Ponds are much less common in this part of the US than in others, so unfortunately there isn't such a thing as a pond management company I can call on for help. Thankfully I have the knowledge and input of each of you on here to help guide me and I'm sincerely grateful.

I saw a pond a couple miles from me on a satellite image and am thinking of dropping by the house next to it to see what there experiences have been.