Is it considered an innovation (bid‘ah) if I instruct that a sermon be delivered at my burial to soften hearts and bring sinners to repentance, knowing that none of the Companions did this?

Admonition and reminding people when burying the deceased is a prescribed matter, and there is no harm if a Muslim requests that an admonition be delivered at his grave, for there is no innovation or Sharia violation in that. It is better for a Muslim to make a will regarding his duties and to be prayed for.