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@Russell-Pirie "...will they all fit??...." You'd need to be specific as to what they are and how they are configured in order to answer that.
I'm not sure you need to rewrite a bluetooth driver, most users would utilise "Bluez" package for bluetooth, perhaps take a look at that package and see if it meets your requirements, here's a link to the package:
https://openwrt.org/docs/guide-user/hardware/bluetooth/usb.bluetooth
I flash my devices using a bash script (well ash), it just connects to the device(s) using ipv6, uploads the firmware and then ssh's into the device and execute the firmware upgrade. You could tweak my script a bit to make it handle multiple devices at once. I did about 40 of them today using my exiting script, one by one while I was eating lunch and watching the news.
My script is here:
https://community.onion.io/topic/4483/script-to-deploy-firmware-to-device-using-ethernet-ipv6
The other option is to boot into u-boot and flash from USB.