I've made Gan-jjajang at home a few times, but stir-frying the black bean paste is the hardest part

It might be different for people who cook often, but if you don't stir-fry the black bean paste properly, it ends up tasting bitter, and if you stir-fry it too much, it burns.
To make it easier, it's better to buy pre-fried Chunjang, stir-fry it with onions and meat, and then add the pre-fried Chunjang at the very end and stir-fry it briefly.
Also, you need lard to easily achieve that authentic flavor.