I love Zephyr. In fact, the RTOS term is sometimes a bit misnomer. I'd rather name Zephyr as a embedded OS framework: you can even use it without scheduling (even though it will be very limited). The amount of pre-built work done with Zephyr is amazing and when we have to use the STM32 HAL API manually we love Zephyr even more. Really, documenting API using PDF should be banned, ST!
We developed the RTOS (Real-Time Operating System) for our activity trackers using the Zephyr kernel and its libraries.
In this article, I briefly shared a short summary of my experience of choosing different tech stacks for the firmware development of activity trackers.