The #1 key: design a template that is simple enough you can send it out on the first of the month, every month.
That way you will actually get it done.
Beyond that, in SaaS, I love a short update that has:
- Revenue % growth this month, and for last 3 months averaged
- Paying Customers, % growth
- Net Churn, change
- NPS, change
- Burn rate, change
And ideally, the average of the last 3–4 months for each, too. More on that here: The Power and Honesty in a L4M Model. Build One Now. – SaaStr
That, plus a few short lines on how things are going in general, is plenty, and better than 95% of startups.
Anyhow, anything you can’t get out quickly, drop from a monthly update and save for a quarterly or board-level update. Get the monthly updates out fast.