You have 4:2:0 in the original- not great for chroma, so you do not want to loose any more color info. You will not get anything more from transcoding as far as the color info goes, but it will be faster to key in AE if you use an uncompressed format. In your situation I would convert to an uncompressed format, key, composite and render to ProRes and then import for edit.
Tudor “Ted” Jelescu
Senior VFX Artist