What are the requirements for EU freelancers applying for a Thai visa regarding proof of income?

You can make life easy or very difficult.

Easy way is to visit the one and only official website

make an account, fill in the questions and when all done, checked, double checked you continue to payments.

And your application is done.

You might visit the website of the Thai Embassy to get some more information about their requirements, processing time and fees/payments.

Hard way is to ask random questions without the place where you are going to apply and without a passport.