>Then you have to still subtract the days you are going to be sick (Google says 18.4 days on average per year)
What? Do I understand correctly that "Google says" and you find credible that on average a person takes 18.4 sick days every year? That sounds high by at least an order of magnitude to me. Even if I took a day off every time I had a slight sore throat or a moderate hangover, I still wouldn't take more than 3-4 sick days a year most years.
There’s a lot of long term sickness that is skewing the average. For instance cancer or a herniated disk can keep you from working for pretty long.
The source of the 18.4 days has a lot of further information, albeit in German: https://www.iwd.de/artikel/krankenstand-in-deutschland-49865...
Then you have to still subtract the days you are going to be sick (Google says 18.4 days on average per year) and the tile you need for administrative tasks.
If you work with a contract your workplace will pay half of your insurances in Germany, if you are self-employed you have to pay that yourself.