Rendering 360 footage for iMovie then YouTube.

Rendering to YouTube using 360 Studio on Mac. Just uploaded a YouTube video here: https://youtu.be/gCDwV3zjidI using my Insta 360 One X2. When I saved each file I used 5120/2880 and 100 mbps. I then imported the files into iMovie and used them in conjunction with my GoPro and other camera footage (4k on those devices). The resulting iMovie file would only render at 1080p and the 360 cam sequences in my YouTube video are grainy to say the least. Take a look at the opening shots and you’ll see what I mean.

The GoPro and my other camera shots are pin-sharp.

any ideas?

Thanks for looking.