One of the most important things to realize about thumbnails is how they FEEL when you are making them. If you think anyone has a "clear" picture of what a thumbnail is before they start drawing, you would be mistaken.
I try to explain the thumbnailing process as feeling like you are trying to describe a dream that you can just barely remember. Like you are pulling it out of the fog. When I'm doing thumbnails I have that hazy notion of what a scene looks like, but I really don't know. The important part is just getting something down. Anything will work for the first one. I don't judge that one at all, I just use it to get me going. Things like "wow, that shot is too wide" or "ugh, that didn't work at all because of this or that" is what makes me try for the next thumbnail. Many times I'll have like 10 thumbnails of even one simple idea. I'm just moving the camera around and changing stuff up. A lot of times I'll copy a thumbnail (working digitally) and erase the border I drew and make a new size out of it so I can add something, etc. This is why I don't like working with pre-made boxes like you have here.
Have you seem my "how to do 50 thumbnails" video? If not, I talk about some of this stuff there. https://www.youtube.com/watch?v=jghVE4V5FfU&t=1s
Keep going!