Hi @MartinEnG ,
Thank you for your interest in the NXP product, I would like to provide service for you.
From your description, in fact, you are very care about the low power.
You mainly need the MCU which can support:
low power
SPI interface
WIFI interface, maybe you can use the UART, SDHC method interface
3.3V power
USB
To these requirement, I think so many MCU chips in the NXP can meet your requirement, you don't need to use the I.MX processor, that is very higher level to you.
You can consider: kinetis L series, LPC series, I.MX RT101x series.
https://www.nxp.com/products/processors-and-microcontrollers/arm-microcontrollers/general-purpose-mc...
https://www.nxp.com/products/processors-and-microcontrollers/arm-microcontrollers/general-purpose-mc...
https://www.nxp.com/products/processors-and-microcontrollers/arm-microcontrollers/i-mx-rt-crossover-...
https://www.nxp.com/products/processors-and-microcontrollers/arm-microcontrollers/general-purpose-mc...
Wish it helps you!
If you still have questions about it, please kindly let me know.
Best Regards,
Kerry