If you do get a handlebar bag- be careful it doesn't mess up your shifting. That happened to me- there wasn't quite enough space and it put just enough pressure on the cables.

I'd favor jerseys with pockets, lightweight raincoat wrapped around one's waist, under-seat bag, or if needed- something that attaches to the seatpost. And you can definitely put an aluminum post in a carbon bike (I've done it).

Have fun on your trip.