No, Johann Sebastian Bach wasn't self-employed. That was not known yet in 1700 in a nowadays form. Plus, it would even have been unthinkable to feed a family, especially such a big one like the Bachs were, without a job on an employment basis.
One of Bach's four famous sons was sort of a freelancing musician for a while. In the time of Johann Sebastian Bach making music was more like a handcraft than like creating art. It served for making money like a baker baked bread or a tailor sewed clothes.
Bach was never self-employed, as we do know what his professions were and where he was hired, except a very short term, after he returned from Lueneburg and before he started to work a first tiny period for the duke of Weimar, Ernst. It's more of the opposite: working for yourself today means, that you have a certain option to chose, who you really want to work for. If you can afford this. And that is, what the most self-employed persons can not afford, as they need to make money with their profession. Actually, Bach was more of the opposite in his second nine year long term in Weimar. The duke of Weimar even owned him. Like he owned all people in the princedom of Sachsen-Weimar. Bach even wasn't allowed to change his workplace or his address, without asking his sovereign for permission before.
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