Write the message you hope you won't need. Unless holds it, warns you before it goes out, and sends it unless you stand it down.
Connect SlackMessages post from your own account, not a bot. Your manager sees a note from you, in the DM you already share.
A notification lands before the send. Tap it, stand the message down, and nothing goes anywhere. Do nothing and it sends.
Every schedule has exactly one message waiting. When it resolves, the next one is booked — no queue to lose track of.