On what they actually say, since most onboarding emails fail on content rather than timing.
One job per email. One thing to do, one link. An email with four suggestions gets none of them done. If you have three things to say, that is three emails, and only if each earns its place.
Address the specific stall. Not a generic nudge, I noticed you have not connected a data source, here is the two minute version, here is what it looks like when it is done. Specificity is what makes it feel like help rather than marketing.
Be short enough to read on a phone without scrolling much. Four or five lines and a link.
Send from a person, and mean it. A reply-to that reaches you, and a sign-off with a name. On a small product this genuinely works: people reply, and those replies are the best product research you will get.
Ask a question in one of them. What were you hoping to use this for is a remarkably productive line, and it turns a broadcast into a conversation.
What to leave out: a tour of every feature, your origin story, and anything that reads as though it was written for a much larger company than yours.