HiHaoMike, I'm very interested in this:
"Come to think of it, since the main (only?) reason for the 120F DHW minimum is bacteria growth, maybe the solution is to use the large heat exchanger to supply the DHW, with some sensor to detect DHW flow and then energize a pump to direct the flow from the 120 gallon tank through the boiler side of the exchanger."
On the Solar tank, it's a drain back open to air (O2) system. The 1,000g (dead) water never enters the house.
The heat is extracted via the 500' of 3/4" O2 barrier pex (in shorter sections to reduce pumping head loss)
I think that I read yesterday that the Building Inspector would not allow my Rheem ST120 tank to be used for DHW, something to do with ASTM labels. So I can either use my 40g hot water heater tank for DHW or buy a bigger one or use HX you described above.
https://surpluscityliquidators.com/view_product/171078/
Speaking of pumps, I bought 3x of these 240vac 53W magnetic coupled impeller pumps, I couldn't find pump curve chart anywhere, I'm wonderering where to use them, possibly 1 for DHW circulation to provide quick hot water at all fixtures in house.
I will mess around with them getting an idea on head & flow rates, most pumps should be operated in middle of curve.