Sorry, new?! Unless this is something different from what I’m imagining, thermostatic shower valves have pretty much been the standard in Denmark for all of my life. You can get them for 80 usd (or probably even lower). These are purely mechanical, no need for batteries. I can even find one on US Amazon for around 50 usd (https://www.amazon.com/Bathroom-Thermostatic-Showering-Temperature-Control/dp/B071ZP4ZHN/ref=ex_alt_wg_m?_encoding=UTF8&pd_rd_i=B071ZP4ZHN&psc=1&pd_rd_w=MLPp3&pf_rd_p=e06954f1-ab26-49e7-940f-21fea3d5decd&pf_rd_r=1YGK573NBS6VC329GYD0&pd_rd_wg=PC4iL&pd_rd_r=4415efd7-7117-45ad-8b60-0a1c86e84ef0&content-id=amzn1.sym.e06954f1-ab26-49e7-940f-21fea3d5decd).
Edit: I realise that this sounded more aggressive than it meant it to. I was simply quite surprised! :)
Sorry, new?! Unless this is something different from what I’m imagining, thermostatic shower valves have pretty much been the standard in Denmark for all of my life. You can get them for 80 usd (or probably even lower). These are purely mechanical, no need for batteries. I can even find one on US Amazon for around 50 usd (https://www.amazon.com/Bathroom-Thermostatic-Showering-Temperature-Control/dp/B071ZP4ZHN/ref=ex_alt_wg_m?_encoding=UTF8&pd_rd_i=B071ZP4ZHN&psc=1&pd_rd_w=MLPp3&pf_rd_p=e06954f1-ab26-49e7-940f-21fea3d5decd&pf_rd_r=1YGK573NBS6VC329GYD0&pd_rd_wg=PC4iL&pd_rd_r=4415efd7-7117-45ad-8b60-0a1c86e84ef0&content-id=amzn1.sym.e06954f1-ab26-49e7-940f-21fea3d5decd).
Edit: I realise that this sounded more aggressive than it meant it to. I was simply quite surprised! :)