Yeah that too, but that has limits, for example european union regulates building industry in such way that every new build, rebuild has to be done in a way that your heating energy requirement is already lower than your hot water energy requirement. Because hot water energy usage can not go lower in current society, but buildings can be improved a lot. So yes as you said if building is modeled in software tools like OpenStudio ( Revit, archicad uses this sw developed in collaboration by NREL, ANL, LBNL, ORNL, and PNNL ) before build, to make building not waste energy and capture as much sun in cold period as possible then even such strategies can be used. You can not preheat/ precool 1870s handhewn cabin, all energy will be lost very fast. It sounds obvious to you and me but most people do not really understand this deeply enough to "click" in their heads.
time of use billing - tool to incentivie you to use "off-peak" power, but i guess it will be deprecated in favor of "realtime" billing in future, because there will be so much solar (almost zero $ per kWh on market) that your energy provider will incentivize you to draw energy during peak solar "activity" AND off-peak hours. it will be simpler for them to give you market price every 15 minutes window than 4hour window at same time every day.
energy generated by big wastewater plants is methane from microbial activity. also waste water plant can not remove a lot of stuff like medicine, hormones...
you can construct wastewater tank with integrated coil connected to heat pump. so you can take all heat back. if you have house with integrated waste water treatment, this should be no brainer. houses with existing heat pumps can "just add another heat exchanger circuit"
but i do not personally like heatpumps because working fluid can be in orders of 10 000 times more harmful to greenhouse effect than co2. and compressors using CO2 as a working fluid are rare.
heatexchangers connected to vertical wastewater pipe are showed in tradeshows. but i do not understand how that makes sense price wise. im not sure they recover as much heat as advertised.
You seem to have concluded energy use for hot water cannot go lower by excluding any approach that would lower it, not because it's physically impossible, but simply because such technology isn't being used.
time of use billing - tool to incentivie you to use "off-peak" power, but i guess it will be deprecated in favor of "realtime" billing in future, because there will be so much solar (almost zero $ per kWh on market) that your energy provider will incentivize you to draw energy during peak solar "activity" AND off-peak hours. it will be simpler for them to give you market price every 15 minutes window than 4hour window at same time every day.