ffmpeg h264: VLC could not decode the format "aivx"
aivx is simply a specific tag for 4K H.264 streams. Surprising that VLC doesn't probe the stream.

Try with

ffmpeg -i myfile.mov -c:a copy -c:v libx264 -preset slower -crf 20 -vtag avc1 myfilearchive.mov


In some cases, ffmpeg may fail to set the tag without warning. Add -strict -2 if that's the case.