Silicon is actually not a good insulator, especially when tight fitting, so has minimal effect on the heat rejection of the phone.
Things like Bluetooth and WiFi draw high amps, so switching those off has a far larger impact.
Battery saver mode, low brightness, dark screen theme, dark background pictures, and things like that will save a lot of power and reduce current draw and battery temperature.
Batteries actually operate more efficiently when warm, so unless it's a 50⁰ day you should be right.
Modern phones use battery cooling systems now too, usually with the cooling plate behind the screen so it isn't covered.
Unless it's a decade or more old, the heat from the core of the phone gets piped to behind the glass.
