Arduino minimal power usage
Arduinos are for rapidly getting something going without having to know much about the underlying electronics and microcontroller.  As a result, some design tradeoffs were made for you, generally optimized for development simplicity.

You now have a different primary concern, which is low power consumption.  If that is truly important, then you have to design the system with that goal up front.  This means starting with the bare microcontroller, designing the electronics around it carefully to minimize power, and designing the firmware architecture to minimize high-power cycles and maximize sleep time.

You are asking where to strap the big engine on a Honda Civic to make a high performance racing car.  You can make a Civic higher performance, but to get really high performance you have to start with that goal from the beginning.