storage tank?

Good post.
One note on EPDM. You were lucky to have it maintain so long. Lower temps do allow EPDM to last for a reasonable life. It does tend to break down and the breakdown byproducts do affect copper tubing.
We have not seen this with other liner materials.
In full disclosure, we have seen some DHW hx failures due to corrosion, usually from the inside out. This seems to be due to pH issues.
We have subsequently shifted over to stainless steel hx for DHW.

I'm using a 500 gallon stainless chemical tank. Upright cylindrical with very thick walls that I got a terrific deal on. I had all the fittings for thermo wells and water connections welded in before putting it in place. 200 feet of 3/4" copper for heat distribution and DHW and brazed plate heat exchanger for charging. Bought the copper before the exchange rate went to 1 pound of copper for 1 pound of gold.