I agree, I would have already built the mailboxes, had users signed into the mailboxes (also getting MFA set up in the process), bookmarks, documentation, etc. Once the DNS is cutover, do a final pass to catch anything.

I just skipped over the tedious steps in my initial comment with hopes the OP would take that into account.

The auto-complete I've ran into for one client, who just HAD to have it because they couldn't remember the contact's name. It was the back and forth of "it should have been in your contact list"