@Meghan These are gorgeous images that you have captured.
The tones and gradations of light and color are wonderful, and very calming to look at, and help me marvel at your subjects.
I do have a minor suggestion, which I hesitate to even mention, but you expressly requested feedback.
On my monitor ( new iMacPro - uncalibrated ) the first and third image seems darker than necessary, and will print a bit dark I suspect. When I measure individual pixels in your images looking for highlight tones, in the first and third images, I find almost nothing brighter than 140, 140,140. In the whites of the eyes of your first image, I measure 122,122,122 or so - barely a mid tone grey in brightness. In the third image, even the white of his illuminated eye is barely a quarter tone brown or grey 67,58,59. I know these are meant to be dark, moody, even brooding portraits, but as prints they may not be as bright as you want them to be.
I wonder if your monitor is set to a brighter luminosity than mine, perhaps; but the measured RGB pixel numbers tend toward the darker tones as I measure them on my screen.
When I measure the highlight tones of the specular reflection in Heisler's portrait of Luis Sarria in @Chris's post, I measure them 249,249,249. and the highlight on his nose 222,222,222 is brighter than the white of the eye in your third image.
I am not a portraitist, but I have spend a fair number of hours editing images over the years, and even though my monitor is not currently calibrated, my screen and my prints are a very close match in color and luminosity. Generally I think most images have a full range of tones from pure white to pure black, but certainly not all.
I am aware and sensitive that the numbers themselves are meaningless, but they do provide a framework of the general brighness/darkness/tonalities of portions of an image, that may help an artist to structure the best images they possibly can.
Once again I want to say how much I like these images and that my comments are not meant in any negative fashion, but only offered as a suggestion that you MIGHT wish to consider based on your intentions for your creations.
@Chris. I have had Heisler's book in my library since it was first published, and now I find myself rereading it again like it was brand new. Thanks again Chris. Great read, and something to think about when I begin to play with the Sigma 105 f1.4 you made me purchase a while back.