You make a good point about being hard to judge in the mirror especially since things change so slowly. Since you’ve already started I would keep taking pictures at regular intervals, whether you want to post them or not. It’s much easier to gauge your progress comparing before and after pictures than just looking in the mirror.
If you’re going for the look in the picture you posted then I wouldn’t follow any of the standard HST workouts. You want to focus on building some (not a huge amount) of size in your chest, shoulders, biceps and abs but not your back, triceps, butt or legs. In other words you’d actually skip a lot of the compound “mass” building movements and focus on things like Flyes, Curls, Front, Lateral and Rear Delt Raises and abs work. You might be able to get away with some Incline Presses but you don’t want to develop big Triceps. Some Close Grip Chin Ups might also work as long as your lats don’t get too big. You do want your lats somewhat wider for that look but not thicker. Definitely stay away from heavy Squats, Deadlifts and Rows as they will make you look more like a body builder.