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

What do you think about steward?
I think steward comes across slightly more like a ‘head servant’ type rather than a title of someone you (as a sub) are giving yourself to. You could maybe have fun with some roleplay elements in that, but for me SFF books have ruined the title as one which I would ‘worship’.