I have also been bashing my head against this wall for a while. The ESP32Cam provided hardware support to cut power to the OV2640 during deep sleep which allows it to achieve the lower current drain it does. Seeedstudio did not provide any means in the Sense design to cut power nor to drive the POWERDWN pin so there are no hardware options open to us. However, the spec for the OV2640 does talk about putting it into a software standby mode by setting bit 4 of register 0x109 to 1. I and others have tried this: see OV2640 standby support · Issue #101 · espressif/esp32-camera · GitHub but it doesn’t work. Perhaps there is some clever bit twiddling that will make it work but I haven’t found it yet. My conclusion is that the OV2640 has a silicon bug that we can’t work around which makes the Xiao Sense useless for battery-powered applications as supplied.
The good news is that the OV3660 is plug-and-play compatible with the Sense connector, has a better sensor and the software standby command works. See OV2640 standby support · Issue #672 · espressif/esp32-camera · GitHub
Taking a picture every minute and putting the OV3660 camera into software standby mode between pics I’m getting a current draw of around 20mA.
Or you can put the camera into standby mode and then deep sleep the cpu for a current draw of less than 5mA which is the smallest I can measure.