In a SaaS product, these are different issues:

1. I am making a proxy for churn (last seen == end of subscription in this product).

2. You are looking for active users (yearly / monthly / ...)

I think your suggestion (point 2) is definitely an important view, but it doesn't conflict with the need for point 1.

Sorry if this is self-evident, I will just leave this link for reference on such metrics: https://userpilot.com/blog/product-engagement-metrics/

In any case, I am happy to help: if you would like an export of the data, or the DB dump, let me know. And I very much looking forward for your analysis :)