Externally supplied hot water through a plastic tube makes this a non-starter for me. You use almost no plastic (wonderful!), but the little there is has a lot of surface area touching hot water.

Have you thought about making a water heater accessory? I'd be open to collaborating - contact[at]otekengineer.com

That grinder is a thing of beauty. I was going to impulsively splurge until I saw the price (not complaining, you're doing the right thing by launching at a high price point).