The design looks good. I am interested in how you get on with it.
I am now thinking of changing the buck convertor to a 7805 or a 7808. I think this may make it a little easier to buy the parts. Wary of constantly changing the circuit though.
If I use a 7808 the power can go to the vin pin then the Arduino can power the Bluetooth module. I had polarity issues with using the 5v pin hence the IRF9Z24N.