People will attribute masculine and feminine gender to ‘master’ and ‘mistress’ respectively, but I think master would be the ‘neutral’ pick here.

Or you can shuck the dynamic with: owner, divine, ruler, royal, liege, or any number of ranks with which you attribute subservience! :3

Yeah I have to second the “master” as a gender neutral. People always think of it as masculine, but there is no feminine alternative for most uses of master. You aren’t your dogs mistress, for example.