comments:
roleohibachi posted on r/askelectronics2w
I think you've described every thermostatic fan controller to every exist. Maybe you were just missing the keyword, "thermostatic", which specifies a device that seeks a stable temperature. An example, not necessarily the best example, not guaranteed appropriate for your exact specs. https://www.amazon.com/Temperature-Controller-Control-Thermostat-Thermometer/dp/B09STXT6T2