One other easy thing to try - by default BigFix actions run in 32-bit mode. I don’t think that should matter for bcdedit, but you could try adding
Action uses wow64 redirection false
To the script anywhere before the ‘waithidden’ command.
Edit - the rules on escaping brackets can be confusing, I summarized them at Tip: Escaping curly brackets for substitutions in ActionScript , please let me know whether that explanation is helpful.