What do you report on to build early internal confidence with the product strategy? And how often do you report on this?

This really depends on the maturity stage for your product.

For 0-1 work in startups (new products, not-yet-released), I recommend the models explained by Steve Blank (e.g. customer development process, business model canvas). There are so many unknowns and intertwined hypotheses for brand new product ideas that all need to be validated, iterated on, and improved until the whole product concept and its acquisition and retention funnels are working. The business model canvas helps clarify the scope of all this and the risk in each area. You generally want to report on new information as soon as it's available, but frequency will vary (e.g. maybe daily for marketing funnel performance, and once a week for learnings from sales calls).

For live products that have active users, I'd recommend:

1. focusing on engaging and retaining users first,

2. tracking these metrics daily, and

3. evaluating your product improvement hypotheses based on whether they improve engagement and retention.

When retention is in a good place, then shift to growth - first by finding more users like your current users, and then by expanding to adjacent and new audiences. Continue monitoring and reporting on engagement and retention, and now also track daily growth metrics.

Your high level product strategy should make it clear which maturity phase you're in and what markers you're aiming for in order to "graduate" to your next phase. E.g. "Stage 1 out of 5 for our product strategy is building consumer retention. When we achieve XYZ in terms of retention, we'll be ready to transition to Stage 2/growth."

You likely also have a strategy for succeeding at the current phase (e.g. "here's how we will out-compete product alternatives to earn people's engagement and retention in Stage 1").