Track metrics at three levels. Email-level: open rate, click rate, and unsubscribe rate per email. Sequence-level: overall trial-to-paid conversion rate, time to conversion, and which email drove the most upgrades. Business-level: revenue per trial, LTV of trial converters vs. other channels, and payback period.
The highest-leverage optimizations are: subject line A/B tests (these affect open rates, which cascade into everything else), send timing (test morning vs. afternoon), and CTA placement (test inline text links vs. buttons). Start with subject lines because they require the least effort and produce the most measurable results.
One often-overlooked metric is the "last email opened before upgrade" report. This tells you which email in your sequence is doing the heaviest lifting. If most users convert after the social proof email, invest more in customer stories. If conversions cluster around urgency emails, your product may need better in-app onboarding to drive earlier conversions.