Are you set to do the code implementation for iOS? And can you work with uniq on getting the Android one done as well? As I understand it, the data points to collect are:
Which backends folks are using. I would consider this to be collecting when a user sets up a new backend (each setup of a backend would count, so if they have 2 Next Cloud, that's 2 events). I would assume doing this on a weekly cadence would be sufficient.
"How much data is being uploaded". I would consider this to be collecting the sum of uploads per week per backend. E.g. at the end of the week, reporting "John uploaded 3 things to NextCloud and 1 to the Internet Archive".
Ofc, y'all know better than I your use case so do what suits you :-)