Ustadh, do you have any advice on learning to speak less and avoiding the kind of idle speech that leads to heedlessness of the tongue?”

The way to it is two things. Guard your tongue from what Allah forbade, backbiting, tale carrying, obscenity, insult and lying, and then occupy it with what benefits, dhikr and recitation of the Qur'an, so it is busy with good rather than left idle. From the greatest ways to help you achieve that are good company and lots of dua.