20 amp breaker/12 gauge wire and a light fixture?

hello GASH ... you are correct in your thinking ... there would be no heat build up to the fixture wires or the light bulb with using the 20amp breaker and the 12/2gauge with a ground is the correct way to go ... if you do not own a little two wire glow lite electric circuit tester they only cost a few dollars and it will help you check that your wiring job is correct before blowing any light bulbs .... === at the light fixture and after you have wires in place you need to cut the black wire and add in a pair of black wires to go to the light switch [[where you want to have the switch and you may not need the switch/light ON all the time but, you may want to use a charger from the outlet and then if your light is not switched it will be on all the time == be sure that you install all the proper in-wall boxes as those are important for fire safety [[[[[[ note ;;; a missing in-wall box is a safety issue and can be a reason for the home owners insurance to NOT pay for a mishap ... those in-wall metal or plastic boxes are cheap buy the one that works for you ....

12 gauge wire can handle 20 amps with no problem. That is up to 20 amps you would pop the ckt breaker if more than 20 amps was used.

THHN/THWN building wire is not tested or approved to bear any weight. Any kind of pendant fixture will have some type of "messenger" material, like steel cable or kevlar.

Any pinch point would be a risk of overheating.