After clearing the Protocol field in stunnel config, which I had originally set to ldap, saving the change, and restarting stunnel service, executing a connection test from the Toshiba MFD was successful.
And after adding the Google Workspace server entry in the Toshiba MFD LDAP Client settings as a directory/service option (click Server Assignment button, also in MFD LDAP Client settings), Google Workspace directory searches from the Toshiba MFD are working as expected.