Onboarding metrics are usually about reducing friction: time to first login, completion of a setup checklist, percentage of fields filled. They are easy to measure and easy to move. They are also weakly correlated with retention, because completing setup is not the same as understanding why your product matters.
The metric that matters
Time-to-aha: the time from sign-up to the moment the user feels the core value of the product, the moment that makes them say "oh, this is useful." That moment is different for every product, and it is rarely setup completion. For a workflow product it might be the first time work moves through it. For an analytics tool, the first chart that surprises them.
How to find your aha
Watch your most engaged users. What did they do in their first session that the churned users did not? That action is your aha. Make it the goal of onboarding, not the setup checklist.
What to redesign
Strip onboarding of anything not on the path to aha. Move setup steps to after the aha, not before. Pre-fill aggressively. The user should reach the aha within minutes, sometimes seconds, with as little ceremony as possible.