I did not think that the capacitor would hold the current peaks of the peripherals, only the RFM peak at the transmission when the take-off rises.
Moteino 8 MHz is essentially Arduino Pro mini without some peripherals (voltage regulator, LED ...). With him I got to sleep 3-4 uA. I also worked with the attached RFM69HW, but I did not get into sleep mode yet because I used a different library. It would not be a problem with your library, I'm about to go through the tests.
If I understand that well, RFM69 is still connected to the power supply (battery). The parallel connected capacitor should cover the transmitting tips. U RFM69W max. 45mA, at RFM69HW max. 130 mA (according to datasheet). And with power management and sleep mode, you're adjusting your energy performance.
In my case, I want to run several transmitters that will send measured data (temperature, weight) once every 60 minutes. So, Atmega and RFM could sleep for 60 minutes (8 second period).
The transmitting distance will be max. 50 meters, rather less. That's why I was thinking about RFM69W.
P.S. in the forum I read about RX, TX. What is going on, I do not focus on it.