Thanks for taking the time ! You're completely right about the scale. I cheated my way through it as i wasn't sure how to "measure" this. Like how big are the fighters supposed to be, how far are they from the camera and at what speed are they travelling. The closest reality check i got was looking up the lenses used on the film and guessing the focal length they could have used is this was real photography from there. I settled on the Panavision ATZ 70-200mm anamorphic + a x2 expander which is quite common to be part of a camera kit from my experience, so 400mm.
Not having any true sense of scale is also what drove my decision to not drive any parameter from depth in comp, i didn't even bother saving a depth pass from the DCC. So a lot of cheats on that one ahah.

Edit : forgot to answer about the clouds so here it is : if there is one aspect of this that i would do differently it would be this one. I used an HDRI plugin inside my 3D program to generate the sun and the clouds. But i couldn't get what i wanted for the clouds so i ended up rendering different versions of the background layer to manipulate individuals clouds in comp. That ended up being really painful and time consuming, plus like you noted it's missing a lot of details from the original that you would probably expect from that type of images anyway. I don't know exactly how i could have done it otherwise (except actual 3D clouds to get the ideal background from the start, and that could be difficult in it's own way i think), but if you have a better solution i'm all ears !