Similar to JC2 I'm not sure if it's even worth doing to main missions alltogether. They don't seem to reward you with anything and the map is fully accessible from the start.
I bailed out on doing main missions in JC2 fairly early on and still had tons of hours of playtime just messing around.
Every region has a
as defense for the region's main base, the only way to disable it is doing story missions if Region 1 is anything to go by. So come to think of it you're kinda "forced" to continue the main story.
Also - why doesn't anything grant me Gears except challenges? What's the point of doing the random missions that pop up everywhere if you don't get rewards? Same goes for picking up audio logs, or lighting shrines.