I'm in the early stages of building a Thread + CoAP low-power sensor and using the nRF54L15-Tag as a starting point. Using NCS, the lowest idle current I can get is 13 microamps, measured by supplying 3.0V at the CR2032 holder terminals. The nRF54L15 itself should be 3 or 4 microamps in this state, and it appears that the BMI270 should consume an unavoidable 4 microamps.
What is the minimum possible current consumption with the nRF54L15-Tag board?