iMovie doesn’t read avc high L5.2 movie

iMovie supports 4k 30fps, but not 4k 60fps. That could be the issue. Also, iMovie prefers H.264 Mp4/AAC format. I realize that you would prefer not to do any conversions, but you could make a duplicate of your video and try converting that to the above format. That way you would still preserve your original. If the format is HEVC you might try converting that to H.264.

Reducing the resolution to 1080p sometimes cures the black screen issue. iMovie supports 1080p 60fps.

The release notes on Catalina 10.15.3 indicate some improvement re 4k editing, at least with the 16in. MacBook Pro, so you might update to that if you haven't already.

I don't know whether FCP would handle the 4k any differently.

-- Rich

According to Apple's bulletin of August 2019, iMovie will export 4k video at 30 fps, and 1080p video at 60 fps, but not 4k video at 60 fps. If you are aware of different and more current information from Apple to the contrary I would appreciate it if you would refer me to it. It might help others who want to know.

https://support.apple.com/en-us/HT205345

Before you say that my previous post is "not helpful" you might try the suggestions that I made, to see if a converted video will work in iMovie at some level other than just the sound. Or, you can continue to try using an unworkable video that is too high res for the app. That's your call, of course. I am just trying to give you a way forward.

You might also try the 30 day free trial of FCPX recommended by thesurreyfriends.

-- Rich

What I have found is that iMovie will knock down a 4k 60fps video to 4K 30fps in the timeline. Try this experiment and see if you get the same findings that I did:

Place a 4k 60fps clip in the timeline as the first clip added to a newly created project. Expand your timeline with the slider that is located next to the Settings button to the right of your screen just above the timeline. Now put your cursor on the clip and, while holding down the "r" key slowly press-drag your cursor to the right. A little frame counter box will appear that counts the frames as you drag. If, when the counter reaches 00:29 frames on the next tick it changes to 01:00 second, that means that the clip is 30fps and will export at 30fps. If it reaches 00:59 frames before it clicks to 01:00 second, that means that the clip is 60fps.

-- Rich

Try the 30 day free trial of FCPX.

https://www.apple.com/uk/final-cut-pro/trial/

This is the UK link to the free trial. You may have to go to your own countries site, I am not certain.