Hi GabsterM,
welcome to the Apple (user-to-user) Community!
Re: "Macbook Pro M3 : Sonoma : Avid Media Composer : external logic keyboard attached.
The keyboard works fine but I can't use the avid mapped function keys.
I have turned off the function keys in System settings but they still wont work after reboot?"
If it is a possibility for you: wondering if chatting with the Support team at Avid Media Composer may help you to use the avid mapped function keys on the external keyboard.
Eg: See: https://www.avid.com/media-composer/learn-and-support
Else the manual that came with the external keyboard may have instructions on how to re-set or map the function keys for your particular needs, or the manufacturer's support team / website info. may be able to help.
All the best :-)