Well done! The first thing that jumped out at me is the right arm. I know it's foreshortened, but even taking that into account, there's some proportional issues. If the upper arm is away from us, which would explain why it's so short, we shouldn't be able to see the elbow. If we can see the elbow, then the upper arm shouldn't be this short. If the right shoulder is retracted, another reason why the upper arm is so short, then we shouldn't see the shoulder. Also the hand position looks very uncomfortable. I tried to do this pose myself and it's nearly impossible. His head/face looks too small for the size of the neck. Then there's the question of what's the focal point? I feel like it should be the face because of the blood and his interesting expression. But what's presented is his left leg and foot, which doesn't seem to be doing anything. His left foot is very relaxed. Also, is he jumping in the air or is he kneeling on the floor? If the former, the right leg makes him look like he's kneeling on the floor, which is not a very powerful position. Your rendering is really good though, very polished.
Gouache is a good training medium for oil, even though they're not at all similar. It's great training for oil because gouache is opaque, and can be applied thinly or thickly, which is similar to oil. What's opposite to oil is it dries very quickly, unlike oil that takes a while to dry. But it's a good thing because it builds good habits of painting in decisive shapes. Oil has a long time to be manipulated so students tend to over edit and don't commit to a shape. Finally, even though gouache dries quickly, if you applied it thick enough, it can be reactivated with oil, so you are able to blend the edges if you'd like. Jeff Watts, Proko's teacher, uses gouache to sketch out an oil painting during the planning phase.
Is the traveler the king? Right now it looks like someone random just arrived. It’s a typical arrival scene. If he’s the owner of the castle, maybe having the castle behind him while he kneels down holding his head works better, with a fallen crown on the ground? But I think it’s a story that’s very difficult to convey in a single image. As a pure arrival composition, the castle can be bigger and the personal smaller. Also, show the head of the horse otherwise he looks like a centaur.
I think it’s perfectly normal. Measuring is never the part that artists enjoy. Over time it should get less tiring, but it’ hasn’t got to be “easy” for me. It’s just I can do it AP faster with less revision, still very taxing on the brain though.
If you do a short text summary it shouldn’t take too long. It should help you consolidate your memory and be able to recall easier in the future. But if it takes you a long time, you should evaluate why it takes you so long and what can be omitted. I don’t take notes for the most part, but it depends on the class. For anatomy, I just watched it, did homework, and while doing homework I watched the video a few times looking for clues. Then later I’ll forget something when I’m drawing and I’ll go back to watch a lesson or a part of a lesson. For other more conceptual classes, I only write something down if it’s something I’ve never heard of and easy to forget, which is not much.
I think they look great! Maybe the brush stroke shape could be more definitive, since your style is very graphic. Right now they look like you just put a stroke down and the shape is whatever the brush shape is, without much of your thought and control. Other than that I love the colors and the facial structure.
You gotta watch out for that! I'm prone to hand/wrist/shoulder injuries, but I've never felt "stiffness" in the forearm. You probably need to take a day or two break, and make sure you take frequent breaks during drawing, like every half an hour to an hour. Set an alarm for it. Where in the forearm are you feeling? What's the posture when you draw, like do you raise your arm to draw on the easel, or flat on your desk? Do you also use a lot of mouse or type a lot in addition to drawing? Sometimes it's the mouse or keyboard that breaks the camel's back. Look up "forearm pain physical therapy" and find exercises that you feel right for your condition.
It’s great that you’re thinking about shape design at this stage. You seem to be on the right track but I think you need a bit more mileage. But specific critiques on these: Your thighs tend to get long. The top side of the thigh (quad) can be a slight curve. That’ll make the thigh shape look more pleasing. The chest is too short in both of the foreshortened poses. Notice how much longer the chest is in the photo compared to your drawing.
I think it’s pretty well done. It doesn’t look off to me at first glance. It looks as good as the two references. if I have to nitpick, the left beard should curve the other way. I’m not quite sure how the first horn on the back works. Doesn’t looks like there’s enough room for it to grow. You might want to move the head a little so we can see some of the upper back. There’s a concept in character design that a silhouette should be clear. When you jam the head, neck and upper back together the design won’t look as clean.