Really simple. If your bombard cannon(s) are in range of a building, you probably want it shooting the building. You only want it not shooting the building if there’s siege you’d micro down first (onagers, trebs, other BBC’s, etc.) and yet BBC’s need to be tended to way, way more than the other siege options because of the order that they engage targets. They will always aggro targets in largely the same pattern all non-Ram, non-Treb units will, by attacking the closest enemy unit.
Just adjust them to primarily focusing buildings in range instead and the intense micromanagement turns into focusing threats but otherwise letting them do the job of making progress while you adjust your army positioning to keep them safe, which is how (I think) it should be.