Doors are fairly easy to paint, with minimum brush painting. Tape off the hinge. Remove locks and put rosin paper flat under. Cut the tape to the hinge edges. Paint 2 times and don't paint thick. I roll the recessed panels first, frame and brush over. Always look out for the runs, esp along the edges. Keep the brush marks with the joints. You can put second coating in less than an hour.
Factory finish may be durable, but that's usually for outside finish. Indoor finish paint is good quality and all the work necessary to hang the doors will mean touch up and you can't touch up semi/gloss paints. Cut the paint over the hinge and peel off. Use blue tape.
I also sand the factory primer, but usually they are already dirty. I prime them again.