One lens is a compromise so I would recommend a zoom. The 24-120 4.o gives you plenty of width and enough length for portraits. The f4 isn't ideal, but not far from the 3.2 I find myself at to get both eyes sharp with a 135. 85 is too short for headshots for my taste, minimal compression. I shoot with the 135 2.0 dc and I like the compression, clearly more flattering, and bokeh is stunning. Lots of folks like headshots at 105 but for me that is improved upon with the 135. It is still short enough to speak with your subject. Sure, folks use 200 or 300, I have even used my 400 mm 2.8 but you need a walkie talkie to communicate with the subject. Literally on a shot with it on a crop d500 and 1.7 teleconverter, so 1000 mm equivalent had me 50 yards away. Got to test the transmission of my strobe triggers. What do I carry, a 50 1.4, an 85 1.4, the 135 2.o and a 70-200 2.8. I also pack a 24-70 2.8 or a 35 2.o in case I want more environment included. I disagree with the 24-70 for headshots, too short. 50 even worse. Remember, it's not the lens length that is determinative of compression it is your shooting distance to subject. We think we look like what we see in the mirror in the morning, so what is that, 2' counter, 2' back into mirror or 4 '. At 6-say 9 or 10 feet, the nose is compressed but not enough to be objectionable. Try standing at the counter, watch the nose and take a step back, then another. Watch the compression. The reason the 135 works for me is it pretty much fills the frame at 6-9 feet and I can communicate with the subject. Shoot at 4 feet and it doesn't enhance or may even distort the subject the other way. Want an example of that, use a 35 and get close enough to fill the frame and watch the nose get huge. I have never had anyone tell me they wanted their nose look bigger. But with the 135 they like the photo but don't know why. If you have a zoom, try head shot test shots at 3, 4, 6, 8, 9 , 12 and 15 feet zooming to fill the frame at each location. See which one you like. See where the nose stops being clown sized. See where the nose starts being obviously shortened. That should help you find the range of lengths you like.