What happens to taxes if you are self-employed in addition to your job?

Hello everyone,

first of all….
Yes, I will consult a tax advisor, but I want to go into the conversation somewhat prepared so that I don't seem completely clueless and I can better understand what he means.

I would like to become self-employed alongside my main job.
I currently earn around €40,000 gross per year and would like to know exactly what the tax situation is.

My current knowledge is as follows:

As soon as my turnover falls below €22,000, I have to pay sales tax. From then on, I'm charged 19%, which I can add to the price.

As far as income tax goes, the "self-employed" and "non-self-employed" income are combined. If I get about 20% deducted from my salary anyway, I'll have to add an additional 1% for every €3,000 (there's supposed to be a list somewhere, can someone send it to me?).

Please correct me if I'm talking nonsense, I'm just gathering all the information.

Is that correct so far?

Let's assume I earn an additional €1,800 per month, or €21,600 per year, from my self-employment. When will this total be deducted, and how much would I be left with, and how exactly is this calculated?

Does it make sense to leave the money in the company account?
You can't deduct sales of less than €22,000, can you?

Thanks in advance for your answers.