I notice that SymPy is a standard package that is installed with Sage: https://doc.sagemath.org/html/en/reference/spkg/index.html
I am wondering what functionality in Sage is built on the SymPy library? Sage seems to have it's own variable system built from source: https://ask.sagemath.org/question/63326/package-that-is-used-for-the-sagesymbolicexpressionexpression/