I'm one of the authors that spent years building SageMath; indeed, the goal of our project is to create a viable free open source alternative to Mathematica (etc.).

There's an interesting discussion of actual attempts to create open source implementations of the Wolfram Language in Wikipedia under "Implementations" (see https://en.wikipedia.org/wiki/Wolfram_Language). The first attempt was 30 years ago by Fateman and he received an official cease-and-desist from Wolfram for his attempts; he did a lot of work on open source Maxima as a result.

I think right now https://mathics.org/ is the most complete open source Wolfram Language implementation, and it uses Sympy extensively under the hood. Mathics was dead for a while when the main author got hired by Wolfram (see https://github.com/mathics/Mathics/graphs/contributors), but during the last year there has been an enormous amount of new work on Mathics.