Hello, Spinal:
The reason there are two different boards in the board Manager is that there are two versions of the XIAO-nRF52840 series: one is XIAO-nRF52840 and the other is Xiao-NrF52840-Sense (with sensors), you need to choose the appropriate version library or something may happen.
In addition to that,If you need to know about low power related issues, you can read this article on the Seeed forum, in addition, there are many similar posts on the Seeed forum for you to refer to and discuss with each other, and the people in the forum are very enthusiastic：
Xiao Sense Accelerometer Examples and Low Power - Products & Technology / XIAO - Seeed Forum (seeedstudio.com)
In my impression, Seeed does not use “Adafruit_SPIFlash.h” in the SPI-related library, but uses its own library “eTFT_SPI” similar library, you can refer to the following wiki about QSPI：
The QSPI Flash for XIAO nRF52840 Sense | Seeed Studio Wiki