The best way to improve your video right off the bat is to improve your audio. This video compares 6 different microphone solutions you can hear them being used in an iPhone video.
I’ll let you be the judge on the different mics. The price range is free (for the built-in mic) to around $500 for the high-end Sony UWP-V1. All the links above are affiliate links to Amazon where you can read and learn more about each mic. But I wanted to provide a video so you could HEAR the difference between all of them.
Hope this helps. Let me know your thoughts in the comments.