Socialism creates a system in which everyone cares about the decisions that other people make.
It may indeed, but care cannot automatically be converted into mandating behaviours.
Socialized medicine, for example, takes everyone's money and spends it on the group's health costs. I, as a dedicated taxpayer, now care when idiots start smoking - I have to pay for their cancer care, and I feel cheated by the poor decisions of others. It is only natural, in this scenario, that I should feel that people should be prevented from smoking in order to eliminate this unfair burden upon me
Really good example. So what might happen is that pressure builds for anti-smoking policy, which causes a rapid decline in the rate of smoking and your concern that the health dollar is being misspent results in not only your own health, but the health of the public in general improving. This has been the experience in Australia, where we do have a first rate public health care system, which incidentally no politician, left or right, who hopes for election would dare threaten to remove.
the system has set me up to make decisions for what others do to their own bodies
Well democratic socialism (and I'm hoping the majority of socialists by now accept the need for democratic socialism) wouldn't allow you to make any actual decision by yourself. It might indeed set you up to have an interest in what other people do with their bodies, and if you were joined by enough non-smokers who share your concern, your combined democratic power could affect the law, which may in turn constrain what other people do with their bodies, sure.
This is de facto slavery
And you were doing so well up to then ... sad. No, de facto slavery is when you get others to work for your profit without reasonable recompense above their basic keep. This is much more like having laws against abortion when enough of the population decide women should be compelled by law to carry a pregnancy to term and force their view through the legislature.
As with all democratic systems what is perhaps required is some legal constraint on how far the law can impinge upon the autonomy of the individual (eg a Bill of Rights). Your interests in what others do may be increased in a socialised, but your ability to realise those interests to the detriment of others, as with any other exercise of power public or private under any other system, can be controlled by the application of law. That's what the Law is.