@R-Fey-Realme Thank you very much for your feedback
It was really helpful to come back to the fundamentals and see where my elements sit on the 3x3 grid!
So, my main point is that John Space is in great danger and he is unaware of it. I want the Caterpillar to look very big and dominating the picture arching over John, and John Space to feel small, vulnerable. Maybe I should have shown the previous step where I played with placements, sizes, and environment. But as I have already sorted the big lines of the composition, the difference between the 3 compositions I suggested are indeed very small.
Basically, my questions are: I have the general composition sorted but ...
- should I make John Space a little big bigger so we can see more details of him or is it too detrimental to the big vs small contrast?
- should I move John Space so that there is an overlap? would it help with giving depth? or should I avoid overlap and have too clear silouhettes?
In terms of the 2 compositions you are suggesting, I think that they are interesting in terms of placement and the flow they create, however they break the big vs small contrast I want to convey... Sorry I did not make this idea clear in my post.

Now with the grid you have added to my 3 compositions, I think I can see things that help my decision. For instance (using the order you've used when posting the 3 images), in Image 1, John Space's head in on the third lines intersection, giving it focus. While in Image 2 and 3, it is the gun that is given focused.
Seeing the images as you have presented them, I start to think that image 3 is working better than the others. To have John Space's head under the horizon line and under the third line strenghen his vulnerability and the fact that he is moving out the panel (into the cave). In Image 2, he looks more confident and heroic (which might be a good thing too). In Image 1, John Space is very much aligned with the vertical third line, which does not help conveying the idea that he is moving out of the frame.
I might be overthinking it
too many possibilities.
In any case, thank you very much for your help and making me think!
PS: You can see more of John Space here by the way: https://www.nodragemillustration.com/webcomics
(Although there is an issue with the level of contrast/colors at the moment I need to fix).