I’m assuming the issues are encapsulated in the idea that you need to specify what events mean a user is engaged and using a product. Is there not a service which you can send user events to (sign in, interaction with a feature) alongside some sort of unique customer ID? Seems fairly straightforward and would result in a fairly good accuracy considering you can track a user over their entire lifespan.
The actual implementation of the retention analysis is not too hard. If you can compute unique customer id, then yes. But how do you get unique id, if say user is not logged in? What if user uses multiple devices? What if user creates new account?
The third party solutions I’ve seen (which is fairly limited), is to embed a code into your product, and they take care of the tracking.
Is there really no plug n play options?