The same day, a short note: thank you, one precise point from the talk, your interest. Not a wall of text.
Write down the open questions. If they come back, you will be ready without reinventing everything.
Do not follow up every day. One polite nudge after the stated delay is enough.
If the answer is no, still say thank you. The market is small. Manner is remembered.
Then go train again. One more mock today beats waiting for the next invite.