Can’t see any place in your code where you send Atmega to sleep. 8mA seem like normal working current if MCU is doing nothing.
Check out Atmega328P datasheet page 48. There is details about how to put it in different types of sleep modes. To wake it up you should use pinchange interrupt (as TPL5110 drv pin is not connected to hardware INT but A2)
Kevin Darrah has great video about deep sleep mode on Atmega