Possibly a 3rd option, isn’t the FX only rated for 2S? Assuming that’s all you run, why not just skip the external BEC and simply run the main battery power to the receiver and HV servo? Of course still need to disable the power from the FX BEC (pull and insulate the red wire going to the receiver).

Nice and simple, only drawback I can think of is if there is a problem with voltage sag caused by the ESC/motor, since it wouldn’t have the regulation stage that a BEC would provide. Maybe throw a cap in to provide some regulation. Either way amp draw for the servo shouldn't be much of a problem.

Thoughts, anybody try it this way?