I’m glad this thread got some discussion going, but as @LostKobrakai said, I’m surprised we went to client/server so quickly! 
Whilst not part of my original question, I’ll likely be using plain old form submissions (over HTTPS of course) or possibly Phoenix LiveView. That said, LV would be of less value in this instance as there’s not much I can validate in the registration form beyond unique email addresses which CQRS makes too cumbersome for instant validation to be practical, and password length/unique characters etc.
Anyway, I decided that for the time being I would roll Credentials into the User schema. Either of the approaches that I listed above comes with caveats and compromises, mainly due to the password hashing in events (as already explained) and the complexity of unique validation of email addresses (mentioned above).
I suspect that in Commanded parlance I’ll be looking to use a process manager so that I can respond appropriately to errors caused by duplicate emails before any other, related events get saved. At this stage, simply combining the two schemas is the most pragmatic solution until I’m more comfortable with ES/CQRS.
