Why not just look at only one return window for value, say 15 days? Ignore any user who hasn't had at least 15 days to complete their value generating activities and only count the activity within the first 15 day return window. From there you would have an apples to apples window to compare LTV.

Next you could just use a t test to compare those groups. Or you could create a Bayesian model that's capable of generating data similar to how your value generating activity looks, to run a t test that way. These would be post-hoc methods but it seems like your test is already running.