[dad911]

Thinking way outside the box, and considering most home DWs do not have a booster tank.....

How hot is the booster tank? Can you bypass it, disconnect the heating element, and still wash?

If you need more heat, add a tankless heater and crank the temp?

If you pull the tank, and take it to a radiator shop (are there any of those left?) they could probably fix it.

Edit: if you clean up the rust, you may be able to braze it, but I have a feeling you're seeing the tip of the iceburg.