Not sure if I understand, but you currently have a device that has UART enabled, and you are trying to communicate with it via BLE? Thus, you want to add a BLE chip, i.e. the nrf52, and then relay the commands from BLE -> UART for the second chip?

Regardless, Zephyr is an RTOS which provides OS functionality like scheduling, interrupt handling, semaphores, etc.

It is most likely overkill for what you are attempting to do. You should instead look at the vendor provided SDK for the nrf52 chip and program it bare metal. The SDK is most likely just libraries/drivers and does not come with an RTOS.