When it comes to the act of jumping in to fill a gap or take on a responsibility, there are numerous synonyms for "leaping in to breach." For instance, you might say "diving in headfirst", "shouldering the load", "taking the bull by the horns", "assuming the mantle", or "picking up the slack." These terms all convey the same sense of urgency and willingness to take on a challenge as "leaping in to breach." Ultimately, the choice of language depends on the context and the tone you want to set, but any of these phrases would adequately convey the sense of initiative and bravery involved in filling a critical need.