It seems the power efficiency can be further improved by enabling the on-chip DC-DC converter.
// Enable DC-DC converter
NRF_POWER->DCDCEN = 1;// RTC initialization
initRTC(32768 * SLEEP_TIME); // SLEEP_MTIME [sec]// Initialization of Bruefruit class
Bluefruit.configPrphBandwidth(BANDWIDTH_MAX);
Bluefruit.configUuid128Count(15);
The average current of my peripheral improves from 0.57mA to 0.35mA. The peak current seems to be lower, from 25mA to 15mA. I do not have tools on hand to see if the transmit power stays the same. Perhaps I can use a sketch to scan the RSSI of the broadcast packets although this method is not very precise.