I want to do almost the same as you only with the difference that my fan use 12V 0,23A. I just got them yesterday. I have already used the RRD extender for the 2 extruder fans.
I wonder if it is possible to use an Arduino 4way relay module (Which i have laying around for years) + external 12V power supply, and then connect it to some aux pins on the Ramps 1.4 ( I have Ramps 1,5) ?
Does anybody know ?