Challenges with date/time range selection when combining snapshot and timeseries data

Tomo_M, First of all, thank you so much for continuing to help me! Our organization is new this journey and I really appreciate your help. Based on your feedback, I am leaning towards option 1 as I have 70million documents a month, it may be cheaper on storage cost.

so I think I am partway there. I have 2 questions now.
First question: I can't see a way to do bucket selector in kibana, I see avg, max, min, sum bucket but not a bucket selector.

Second question: in my query I have the list of users that meet the bucket selector criteria... now all I need is count of those users so I can display it in kibana. How can I do that?

Once again, Thank you so much!