I’m just using Mozilla’s Multi-Account Containers extension. In my work’s infinite wisdom I have a total of five “single sign on” accounts. So I have different containers for each account so I avoid the endless “which account would you like to use” and “this account doesn’t have access to this resource”.
The extension allows me to set specific domains to always open in container X. That covers 90℅ of my use cases. Some sites I need to use different accounts with and for that I have to select which one to use each time.
When I migrated emails last time, I setup my old email to automatically forward to the new email. Then on my new email, I setup an automatic label for any email that was addressed to the old address. Every week or two I’d review what was sent to it and either update the email address used or unsubscribe. Eventually it got to a level where I wasn’t getting much at the old email anymore and finally deleted it.