Can't see the forest through the trees is one possibility. One dictionary defines the idiom as an expression used of someone who is too involved in the details of a problem to look at the situation as a whole. The forest refers to the "more important thing," and the trees are the less significant details that get in the way. This idiom indeed "describes a situation whereby superfluous things detract from other more important/prominent things," but I think you might have trouble working it into your context.
Another related idiom is don't throw out the baby with the bathwater. This is used when someone is rejecting an overall good idea because of one unappealing detail. The baby is the "more important thing," while the bathwater represents the less significant details. However, this idiom usually works only when an idea is being rejected, so it may not be a good fit for your specific question.
Although a bit bland, you might also consider, "We need to focus on the big picture," rather than the "petty details," and avoid "getting lost in the weeds" or "drowning in the details." (These expressions need to be combined, though, to get the full meaning of what you're trying to convey.)