From https://www.mediawiki.org/w/index.php?title=SUL_finalisation&oldid=1094046 , next step for bug 39817:
Iterate over all unattached accounts with email and, where all accounts for a username have the same (confirmed) email, merge them all. == Run migrateAccount.php without --safe and without specifying homewiki in userlist.
(Note: this skips usernames used by multiple emails across wikis; only with the --auto argument a winner is selected in such cases, with the same criteria as Special:MergeAccount uses.)
This only needs the list of usernames which in https://www.mediawiki.org/w/index.php?title=Admin_tools_development/SUL_Audit&oldid=1073768 are counted as "with e-mail 3,350,676" (or even just the "non-attached accounts 4,242,868", those without email are skipped).
This is totally non-disruptive and it's even less than what we now do automatically on login, but those 3M accounts have mostly not logged in for a long time. We really need to know if after this cleanup we have millions or merely thousands unmerged accounts, otherwise it's impossible to tell the order of magnitude of actual conflicts there are.