Foreign Income Exclusion + Self Employment

I worked abroad for several years as a teacher in public schools, qualifying for foreign income exclusion by physical presence. Although I worked for a school as an employee, I filed it as self-employment income because employer info did not fit the forms very well. Every year, Turbotax showed my tax liability to be zero.

 

In 2019, I quit the school I was working for did private tutoring for several months before moving back to the US. I no longer qualify for the physical presence test, so I started doing research about tax filing an suddenly self-employment overseas entails 15% self-employment tax? Is this real? How has this never come up in years past?

Additionally, I did not keep precise records of my income, and it was all cash. I do trust the banks of the country I was living in, so I did not make deposits. How do I report income without getting in trouble when I can only estimate my earnings? Thank you for any bona fide response to my questions.