Before saying too much there’s actually a terrific guide for sustain tanking on the forums in the orochi section that you might want to look at. I think it focuses on hammer main as this is perhaps the easiest style of tanking starting out. Personally I started hammer/chaos and transitioned to chaos/hammer since I enjoy chaos more as a weapon. As for chaos/shotgun that is a fairly challenging combination for sustain tanking. So at the very least you might want to start out chaos/hammer and then experiment with other combinations as you progress.
Link for your reference Sustain Tanking