Even though I understand the argument for theists being happier than atheists because they “have” an objective purpose in life and don’t really have to accept the consequences of their actions because of the “will of god”-argument, that’s nothing more than another way of saying “happy to be a slave”.

You're right, it does mean they are a slave. Slave to other people's ideas about how they should live their life. Some of those people claim they are just following Jesus or the Bible but in most cases that is such a preposterous claim it is ridiculous. The Bible is so chock full of contradictions it is impossible to follow it consistently - there is no consistent way, period. Jesus was such a penniless hobo that you'd need to live like a Buddhist monk to come close to following him. Even the Amish have way too many material possessions. Although being farmers, carpenters, and pacifists they are better people than most at it.

Anyway, I digress. Enslaving your thoughts and actions to some rule book may make some people happy, and it's not just religion I'm talking about. Some people claim to be happier in jail, some people claim to be happier in the army, and there is the claim that some literal slaves that are happier that way. Follow the rules you know what happens, don't and you know the consequences. But like all systems there are bullies and exploiters with power in those systems that pray on the weak and usually the system will do nothing to protect you.

Personally I prefer to be a free man. An emancipated mind free to use its powers to the fullest extent. I prefer society to be organised for the benefit of emancipated humans - this is a secular humanist society. I don't know if I'd go so far as to ban mental slavery as we supposedly did with actual slavery (arguably now we have wage slavery to replace it) but I would want to ban mental battery, kidnapping and extortion. People should join a religion free of coercion, in full knowledge of the facts and claims, and with no mental entrapment like "leave or speak about what we do and you'll be shunned or harassed forever". No claims that cannot be backed up should be allowed - promises of healing and health or wealth should meet the same truth in advertising standards that regular products and services have to meet.