While it’s a lot less energy efficient, once saw a video of someone that replaced one of his tank-based residential water heater elements with a combo 240v/24v

While it’s a lot less energy efficient, once saw a video of someone that replaced one of his tank-based residential water heater elements with a combo 240v/24v element and attached a panel to the 24v element with a temperature-driven cutoff relay. Set the tank to 50C or whatever and the element to a higher point to dump extra heat when possible without getting too hot. A very cost efficient approach that probably gets you some double digit reduction without a lot of risky plumbing or freezing issues.