Progress and Characters have not been lost, they are tied to the Steam Account you played on.

Steam Account A owns Last Epoch Steam Account B played Steam Account A's copy via family share.
Steam Account B can still access the characters on their own copy.

Unfortunately there is no way to move a Last Epoch account to a different Steam account, however the account can still be accessed as explained above.

The only data that can be moved between accounts is MTX and Epoch Points, though this requires proof of ownership of both accounts the items need to move between.