Customer Success KPIs and Metrics for non-SaaS Business

Hello All,
I'm looking for some advice from this awesome audience

The software company I work for, Zerto, offers both a subscription and a perpetual license option to customers. The vast majority of customers purchase perpetual licenses, leaving a very small segment of customers that have purchased a subscription. Subscriptions are renewable at the end of 3 years. Perpetual licensed customers renew their Maintenance & Support contract on a yearly basis.

While the core product is non-SaaS, we do have one SaaS component of our product called Zerto Analytics. This can provide us with some insight into customers' engagement with the product, critical failures that occur, and other alerts/events.

For those of you that have worked/work in these types of businesses, what Customer Success metrics/KPIs did you track?

Thanks,

Daryl