The difference between being a slave for a despot and being a bondservant for Christ is that we still remain free moral agents while being a bondservant for Christ. Jesus doesn't want us to serve him out of compulsion, he wants us to serve him out of love, do what he wants us to do because we want to do it, not because we are forced to do it against our will.
There have been slaves who have been offered the option of going free and being on their own but because they have grown to love their masters they have opted to stay and be a love-slave, doing the work assigned to them because they want to do the work, not because someone is hovering over them with a whip about to whack them if they don't get the job done.
We get there by actively, not passively, presenting ourselves to God in prayer, and letting him go to work on our souls.
Doing the will of God slavishly might be necessary at times when asked to do some real tough things that require agonizing death to self, like what Jesus went through in the Garden of Gethsemane when he was pleading with God to let the cup of suffering pass him by. But the ideal is to have a will that has been transformed and a mind that has been renewed so that we actually want to and look forward to doing what we are asked to do, even do those things instinctively without having to be asked.
God changing us from the inside might not sound like being a free moral agent but if I hate broccoli, and I ask a surgeon to replace my tongue with one that loves broccoli and he does just that, I haven't lost any of my free moral agency at any time. In fact, I have gained more freedom because the next time I eat broccoli it won't be just because I know it's good for me, I'll eat it because I love the stuff and can't wait to get more of it.
The freest people on the planet are the ones who do what God wants them to do because that's what they want to do. Freedom is having nothing inside that keeps us in bondage to wanting to act contrary to the will of God.