Sanden do seem to be the recommended option for this. The problem is if you have a functional gas, the economics rarely stack up to change. If I knew for sure the Sanden would last 10 years yes, but unlike very simple gas water heaters, heat pumps have a disturbingly high failure rate.*

*At least that was the way it seemed from my research, similar situation 6.66kw solar on roof, functional gas hotwater. We do have both Gas and Heat pump house heating/cooling. Heat pump is much cheaper, gas is much faster, even running the largest size possible ducted heat pump split system. I have the gas warm the house from 6am to 8am, then heat pump take over for the rest of the day.