[volkandkaya]

They just kept adding features to the point where it became impossible to run on a "simple" stack.

Is there any open source alternative that focuses just on product analytics?

[XCSme]

Not open-source, but self-hosted, check out UXWizz[0], I try as much as possible to keep the features count to a minimum and work just on improving them instead of adding new ones.

[0]: https://www.uxwizz.com/