TL;DR: Just as a proof of commitment, I could give you an initial binary for you to test in a couple of days, free of charge. Read further, if interested.
Good day. I have plenty of experience designing MCU-based embedded systems. I've worked at Freescale Semiconductors for half a decade as an engineer. I've worked with several arm-based microcontrollers (NXP, Atmel (now Microchip) and STs families) and, lately, with the esp8266 and esp32 family of microcontrollers, either with or without an RTOS.
I have designed similar projects before, designing firmware and bootloaders for MCU-based systems, so I have plenty of ARM cortex-M code available which can be adjusted to your particular requirements. I estimate that in the stated time we could have a final prototype, taking into account a long enough testing phase.
Regarding payment,of course, none is required before a proof of work (PCB design ,schematics and other related documentation).
I'd find quite useful for the both of us, if you'd find the time to talk a bit further regarding your project's requirements. No strings attached.