What if I am employed and want to become a flat-rate taxpayer?

Employed people can also have their own flat-rate agency.
The difference is in the contributions you pay:
If you are employed
In this case you pay tax on income from self-employment, while certain contributions are already covered by your employment.
If your employment ends but the agency remains active
You need to submit tax form PPDG-1R with basis 7 – Change of insurance basis. After submitting it, expect a new tax decision according to which you will continue to pay taxes and contributions.
If you have ended your employment
You need to apply for social insurance at the PIO fund branch, and then verify your health insurance.
