I don't think it would make sense to compare some codecs at 4:2:0 and some at 4:4:4. Unless they can all do 4:4:4, you have to pick a subsampling that they can

I don't think it would make sense to compare some codecs at 4:2:0 and some at 4:4:4. Unless they can all do 4:4:4, you have to pick a subsampling that they can all do and compare them at that so it's like-for-like. If your goal is to demonstrate 'JPEG can deliver better quality than this format by using 4:4:4, because that format can't do 4:4:4', then that's great, but it's a different observation than the one they're trying to make. (Also, I would be pretty curious about whether the different subsampling produces superior file sizes.)