You got a lot of users. I have close to 2000 and API is also the key to user audit for me. Unfortunately, the UMAPI is so limited and doesn't have this information, and the API to get product profile details to make UMAPI really useful.
For Adobe Analytics, you can use the 'users' endpoint of AA API itself, https://analytics.adobe.io/users, which will give you the 'createDate' and 'lastAccess' which are the information you are looking for and I am also using to keep track of the usage of AA accounts.
However, I failed to find the same for Adobe Launch and the closest thing is the audit_events endpoint of Reactor API, https://reactor.adobe.io/audit_events, which gives you the list of audit events with details on who makes what changes in Adobe Launch, from there you can roughly deduce the last access from users. Of course, if the user only logged in but made no change, there is no trace.