I am shooting 5184X 2912 pictures sequence to be adjusted in Light Room and edited to video 4K 50p in FCPX.
The question is simple:
Should I make a project 3840 X 2160 or keep the original size?
Doing some cropping/movement inside the frame will have the same effect on the quality?, or it would be better keeping the original format?
I would think it depends on the size of the delivered product and it sounds like you'll deliver it at a smaller size than the size of the raw material.
I would keep it at its natural state for as long as possible and only trim and compress as needed at the last moment. My thinking is that remapping the source to a smaller size reduces quality and then compressing for delivery will reduce quality a bit more.
So, while I can't say definitively not to downsize now I can only suggest to compress once for delivery and not introduce more compression in the workflow. Just my opinion.
The most difficult transition you have To make creatively is transitioning from your original still frame dimensions to what most likely will be a HD 1920 x 1080 framed movie. Taking a step back, when I shoot the timelapse scene I generally construct my frame so that it is wider than what I think the final frame will be .
Then I set up a custom projec in FCPX , usually around 4K depending on the pixels dimensions of the still frame.. Once that is done its easy to do some re composing, pans and scans and moves inside of the original still frame dimensions. Motion can be you friend here to help devise a traveling crop of the original still frames into your final, presumably 1920x1080 HD frame size.