Thanks @PJ_Glasso, your example was really helpful. I was able to get the P25Q16H working with LittleFS on the XIAO nRF52840 using SPI (not the QSPI peripheral). I ran into an issue using this method so wanted to add that here - I was setting up SPI as in the example but also using an I2C device at the same time. The BSP sets up TWIM0 with Wire, and you cannot use TWIM0 at the same time as SPIM0, according to the nRF52840 datasheet, section 4.2.4 Instantiation. To get my project to work with both the onboard flash and an I2C device connected to the standard SDA/SCL pins, I changed my external filesystem to use the SPIM2 instance.
SPIClass SPI_2(NRF_SPIM2, PIN_QSPI_IO1, PIN_QSPI_SCK, PIN_QSPI_IO0);
This works just fine for my project, so I haven’t spent any time figuring out why I wasn’t able to get the QSPI peripheral to work with the P25Q16H.