I don't see why that idea would be expensive though. It seems like something that could be done for $20, and perform as well or better than a large plate exchanger. The biggest downside is the floor space and low SAF.
Copper loops, finding a hot water heater in decent shape, rigging up some water tight seals. I mean remember this isn't a science experiment. It is the water my family drinks. It doesn't seem worth it. Like I said it would provide marginally more recovered energy but hacking around with drinking water isn't my idea of a good ROI%. I also don't think you are going to find all the parts, supplies, connectors, and tools for $20 but even if it is was $0.00 I don't think messing around with drinking water is a good idea.
I think you are missing the point on the amount of water. 2 ft, 20ft, 20,000ft it doesn't really matter how much energy is in the loop. We aren't storing it for on demand use. That energy will be dumped out of the loop at the same rate regardless of how big the loop is.
Your points are valid, it would be significantly more work to ensure it was built well and safe. As for the amount of water, to efficiently transfer the heat you need a large amount of surface area. To have a large surface area without excessive restriction, you'd need a large volume. That requires more water with more stored energy.
I'd actually be very interested in your calculations for the plate heat exchanger you looked at, and what the stats are for your system in terms of flow rate and head pressure, temperatures and such. With 6kW, you have a lot of leeway for heating water, but looking at my own supply even in the summer the incoming cold supply is around 10C. Getting it to 30C requires a lot of power.