“New users” on an event-centric index ticks all the boxes for problems:
If you create an entity centric index for users which holds the date of the first sighting you can run a “new users” efficiently with a simple date histogram on that firstSighted field.
To get “returning users” hit your event store with your example query that had cardinalities and subtract the “new users” value for each day (this may require work in your client)