I would first look into comparator modules and research them. Ultimately, you're looking to create triggers. Once the volume of your contact mics reach a certain threshold, you want to go from a state of "off" to "on." Volume might be trickier. An envelope follower driving a VCA sounds like a good place to start. You may also want to consider a low-pass gate with the contact mic opening and closing it.
Though, it'd probably be cheaper to use MIDI drum triggers and then convert the MIDI data to gates and the velocity to a CV output. Reinventing this wheel in pure Eurorack is an expensive way to go.