How does your footage look straight from the microSD card? Did you have your camera video settings properly set to capture your footage in 4K or at least in a high resolution and frame rate?
I'd work backwards. Take a look at your raw footage on the card. If that looks good then your camera was properly set up. Compare that quality to your output via iMovie. If there is a difference after editing, then that narrows down on your problem, i.e., something went awry during the editing or with how your footage was saved afterwards. These things can be tricky at times. Others in this forum who know more about these matters than I do will probably chime in.
I wish you success on your project and hope you can resolve your issue soon. I know it's frustrating when you know what you are doing but things simply aren't cooperating with you. Good luck!