You have a 60 and a Covenant, and you have opened the Maw and Korthia, so you and your alts are clear to go:
To unlock Zereth Mortis, you must work through Chapter 1 of the Eternity’s End, Into the Unknown. You should receive the first quest for the 9.2 Campaign, Call of the Primus, the moment you first log in on Patch 9.2’s launch.
There are currently no underlying requirements to unlock the Zereth Mortis storyline for alts. Any alts at any point in the Shadowlands campaign are able to start the questline.