Start tamping your coffee. Without tamping channels form a lot easier, making your shot pour a lot faster and making your coffee taste sour and bitter at the same time. If you still manage to get 11 bars of pressure somehow without tamping, I'd assume your coffee is ground way too fine anyways, making it even more likely to form channels since ...
Turns out, it's not heat which is the enemy of good coffee, but too much heat.
The best coffee warmer is one that keeps your coffee at a controlled drinking temperature. The problem with a thermos is that it will keep the coffee too hot for comfortable drinking initially. Reheating will burn the coffee.
So anything which keeps the coffee at around 50°C will ...
Heat is the enemy of coffee.
I brew my coffee in a French press and then transfer it into a carafe, the carafe keeps it from cooling down.
I only pour out 1/3 to 1/2 of a cup at a time.
No unnecessary use of electricity.