I have to agree that shutting off parts of the train to maintain social distancing for pass staff is not a massively good look, but one that is nonetheless necessary. Staff don't really have the luxury of choice about travelling on a particular train and trying to identify particular services which might be quieter or safer was just too much of a headache.
We received suggestions from one of the unions during the height of COVID (when everyone was still bundling onto coastal trains) that we could lock out one coach or part of a coach for the purposes of allowing pass staff or key workers to travel in a socially distanced fashion (and to stop it looking like an empty coach with one member of staff rolling along in it).
However, we ended up finding that wasn't practical - a lot of people made very dubious claims about being key workers while attempting to travel for free, and it would be incredibly difficult for staff to enforce, especially on DOO routes.