A simple heat exchanger from the drain to the cold water running to the shower. A thermostat on the shower reduces the amount of hot water you use as the cold water gets warmer.
Recovering water from the drain to go into the shower again, probably via reverse osmosis.
Second option requires maintenance and a bunch of engineering. Unless there's a massive lack of water where you live and you have lots of money.
First option... Well, it requires more piping and that the drain pipe is a heat exchanging pipe - a pipe within a pipe where the cold water is in the outermost pipe, running in the opposite direction of the drain water. I'm not sure how much heat you would recover. The floor of the shower might need to be a bit higher than otherwise. And if you have hard water then I wouldn't do it. When you heat cold, hard water the calcium carbonate precipitates and you get limescale. To avoid reducing heat exchange efficiency and avoid clogs you would have to descale regularly and it's just not very accessible. With normal descaling you can remove a lot mechanically but here it would have to be all chemical. And how would you even get the descaler into those pipes?
Tl;dr don't do option 1, only do option 2 if you have very soft water.
From my reading of social media there are two kinds of neurodivergents: 1) hates showering, most unpleasant experience, sensory overload 2) loves showering, could live in the shower, sometimes exits the shower and realises that they forgot to wash their hair or soap up or something. Only misses their daily shower when they are very busy and can't find an hour in their schedules.
You may be a neurotypical if you:
shower each day
complete your shower in less than 10 minutes
don't forget any of the 1-3 things you are supposed to do in the shower
Gardening is nice because I can scale it down quite a lot (at least where I live) and when I forget to water or fertilize I'm "encouraging resilience" in my plants.
Some people have romantic partners they live with. And sometimes, rarely but sometimes, those partners have friends. I just think this is a shitpost though.
If it's reverse osmosis literally only water can pass through.