I had actually missed some pin settings combinations... so here is what works me, but be aware that it goes against the excellent recommendations by Nick Gammon (https://www.gammon.com.au/forum/?id=11497) so use at your own risk.
I get back down to 4.5uA while sleeping by enabling the pullups on the A4 & A5 analog pins (after calling Wire.end() or clearing TWEN in TWCR otherwise this may well have no effect):
Wire.end();
digitalWrite(CTRL_PIN, LOW) // power off the sensor
digitalWrite(SDA, 1);
digitalWrite(SCL, 1);
LowPower.powerDown(_sleepTime, ADC_OFF, BOD_OFF);
digitalWrite(CTRL_PIN, HIGH) // if you powered off your sensor
Wire.begin();
delay(250); // let I2C stabilize
Franck