It would be great if there were a PUT endpoint for username. The use case is sites like mine which transitioned to SSO, to get all the usernames synced. Then my plan A would have just worked.
My fallback plan I think is going to be to add another column to store discourse_username also in my DB along with discourse_id. This way, I can directly to their profile with username. The downside is that this username can change. So I need to daily scan all users to re-sync.