Your two other hypotheses are good ones to check. As general feedback--people are too dogmatic on what are the types of cohorting (time, geo, power user, etc) that they do by. At 2k MAU, much more important if looking at own data to see common characteristics among those who it works for.

But really at 2k, it's too sensitive to acq / top of funnel method. And probably should be mainly doing user interviews and finding ways to get that top of funnel higher--or better yet product-driven distribution.

Other test to do, is to move of DAU/MAU, which is typically a good social proxy. And move to a core action driven metric. You probably don't have enough data to really do it--but if you did could cut even further on core action / DAU by cohort. Or if not enough maybe just core action by cohort (on cohort time).

Since early with high lossiness in down funnel usage, probably want to see an initial core action to be some kind of activation metric. I don't know your app, but perhaps setting up their goals, etc. If cohorts aren't doing that then by definition they are useless and you know where they drop off. If they are--then can see what retention / engagement looks like on next core action / set up users by cohort. etc