I bought a piece of 1.5 inch stiff foam to try to fix a sag in a bed. It didn’t work but having that thick piece of solid foam around has been a life saver.
Need something flat to put a laptop on? Throw it on the foam. Going to be doing something that requires you to be on your knees for a while? Get the foam!
It went from stupid purchase to something I’d gladly replace if it broke.
Now that you mention it, I recall that some products could tap into the hot water, but for at least one of the bathrooms that would be even a bigger PITA because the hot water lines are on opposite walls. Also with a tankless house, it can take a while for the hot water to flow, so the last thing I want is cold water flowing for the first 2 mins in the middle of winter. That doesn’t sound like fun.