Has anyone replaced a hot water tank with an instant hot?

Tankless heater doesn't seem practical for the application. Whether your overall goal is conserving water (not just hot water) or weight, how much could it possibly save?

Plus sounds like you don't even use or need a water heater based on your post. Why lighten your wallet?
Idk how resilient they are to freezing either. Regular water heater, drain it, get most of the water out and you're fine.