can we use a smps to light a 15w bulb?
A 15W tungsten bulb draws 150W on warm-up.
15W LED bulbs do not have this problem and are much brighter unless rated in equivalent power.
NEMA 23 is just a 2.3×2.3 inch (58.4×58.5 mm) faceplate.

This does not define the length which affects the current rating and your power consumption P=V*I per 2 windings.(max) although 4.2A is pretty common for the long 2LxW ones.
thus 8.2Ax12V=100W per motor or 200W total
leaving 40W to spare if it is an LED bulb only.




You may need thermal design changes to reduce temp rise in the motors such as reduced current on hold or micro-stepping with less torque.

I suggest a same size  1 W muffin fan if you are using 100W all the time with full steps, running at half speed as the air velocity is what counts.

Additional info

I highly recommend GRBL Panel over windows to a CNC shield on an Uno with uploaded flash using Gcode sender.
then optimize the parameters for optimal max acceleration, velocity to suit your mass, torque and seek times without skipping from 0 to 1m in 1s with 0.x mm resolution