Stepper Motor angle after gear
Am I right in thinking that when the stepper motor is unloaded, each
step (1.8deg) gives a torque of 2.7 Nm?

No. When the motor is unloaded gives exactly zero torque. It would however output a torque of 2.7 Nm when loaded with the same - third Newton's law of motion.

If that's the case, using the current gear ratio, would the output
torque be 2.7 Nm x 4.5 = 12.15 Nm?

Yes. Usually the torque is declared for full step operation and usually it's a holding torque, which is much higher compared to the torque at nominal speed, where the AC motor has its nominal torque declared at nominal speed. You have also to include the efficiency of the transmission. The belt transmission has almost 0.9 (90%) efficiency if high quality bearings are used.

Secondly, if I was to use microstepping (32), does this calculation
make sense?
1.8 step angle/ 32 microsteps = 0.05625 deg

It doesn't make sense at all. With the micro stepping the torque will decrease for a factor of 0.707 compared with the torque produced in full step mode.
P.S. :
Almost every hobby engineer neglects the datasheet of maximum permitted radial force of the motor, when doing belt transmission. With a small diameter pulley, this is exceeded very soon.