Hi @R Neil Haugen
In your precedent message you said :
"The only way I know to approach full lossless with that format is to set the level and Mbps way, way up, and then you can mostly sort of make it ... "
So you are talking about "Levels" and "Mbps".
I have tried to find where is the "level" setting, and I have found this (see blue arrow). Is it correct ?
This level is actually set to 3.1. Is it too low according to you ?

But I do not found the "Mbps" setting. Where is it ?
And in your last message you are talking about "color data". I have seen that the color data of my source video is "4.2.0". So if necessary I will have to go in "Lumetri color" in order to better the colors : Is it correct ?
First, yes, that's the levels I was talking about. You'll want at least 5.1.
Above that is the Profile, and again, the lower options in the drop down are higher capabilities.
The 4:2:0 designation is how much color data is kept in the file. You would do well to look online for information.
4:4:4 is full data for brightness and color data. 4:2:2 loses some color data to save file space, but our eyes can't tell the difference for color, as we are far more sensitive to minor brightness issues.
4:2:0 drops more color to save more file space, and is low enough on color data you cannot do much tonal, color, or saturation changes without creating obvious banding on smooth surfaces or macro-blocking.
And once the color data has been tossed, as in going to 4:2:0, you can't get it back. So your original files are pretty low on color data to begin with.