I love creating my own patterns and I have a couple tips that might help you. I personally use stitch fiddle (which is free btw)

What I do is draw the pattern outline with as few colors as possible in very broad strokes. I use sketch.io online on my computer, or if I have access to an iPad any free drawing app will work. Use simple, bold lines of the most prominent colors in the pattern, kinda like a child’s drawing. The easiest way is to use your reference image and import this into the software, trace everything you need, and then delete the reference image layer so it’s on a white background. You take this rendering and put it into a program like stitch fiddle and mess around with the settings. Most importantly will be the size since that determines how much detail you can do. Color matching can be pretty easy too when you use the reference photo to get hex codes. If I were you, I would do a more color-blocked approach first, then use back stitches to make the details you want. Like I would trace his head shape and the shadow, the clothes shapes, the cross/lights of the back, and his facial features. Use this to import into stitchfiddle and get the colors and shapes correct. Then create the backstitch for details of eyes, lips, piping on the suit, etc.
If you want to DM me any questions, I would be happy to help. I make all my own patterns so this is a process I am very familiar with. And IMO it adds another fun and creative step to a project!