If I can’t use aux-1, that means using aux-2 through aux-5. Since aux-5 and aux-6 are used as relays, does anything have to be done specific to aux-5 to use it for a Dshot connection?
It just means you have to configure the ESCs using a different FC or an Arduino.
Probably the only thing you really have to change is LOW RPM POWER PROTECT = OFF.
You can change motor spin direction in ardupilot (there is a SERVO BLHELI reverse bimask) or the old fashioned way of swapping motor wires around.