Removing a device from Android Device Manager for a specific account
Actually, it is very straightforward. 
Remove the work Google account from your phone and then add it to Gmail app as an IMAP account instead of a fully functional Google account.
If it is added as an IMAP account, it doesn't get linked to ADM or any other Google app, for that matter.
From How to add Gmail account without adding Google account on Android - Gmail Community

Instead of adding the account as a Google account, add it as type Personal (IMAP).  That way only Gmail will be connected.  The settings are:

Incoming servername:imap.gmail.com
Port: 993
Encryption: SSL
Outgoing server name: smtp.gmail.com
Port: 465
Encryption: SSL